The Attribute Combine is great for times when you want to make a new attribute that's the result of combining two existing attributes in some kind of way.  It's also great for applying an operation of some kind to an existing attribute in order to generate a new attribute.  You can also overwrite existing attributes with the resulting value(s) by specifying so in the destination field.  In practice, it's a good idea to keep all the attribute types the same.  So, if you'd like to generate a new primitive attribute, then make sure you're feeding in primitive attributes into this node.

Source Group & Group Type:

--  This allows you to specify which group of points, prims, vertices, or edges you'd like to do the operation on.

Destination:

--  In short, this determines the new attribute to create and/or the attribute that you'd like to override with the combination function.  According to the docs, "Name of the destination attribute to write to. This can be an existing attribute, in which case its size and type is maintained. It can also be one of the attributes used as a source.

--  If the destination attribute doesn’t exist, a default version of the first attribute will be used. This behavior changes depending on the Create Destination Attribute and Create Missing Destination as Scalar parameters."

--  A visualization button is available next to this field if you'd like to visualize the attribute in the viewport.  You can also specify what kind of attribute it is through the dropdown menu next to the visualize icon.  Keep in mind that, "All the source attributes must be of the same class." meaning that if you're trying to do an operation on points, then make sure the dropdown also says points.

Number of Combines:

--  According to the docs, "For each combination the same layering process is performed.  The current value of the destination attribute is read. The source attribute is read and processed with the scale, add, and process parameters to get a source value.  The source value and destination attribute then combined with the operation to get a new destination value. Finally, the destination value is blended into the destination attribute using the blend scale and blend attribute."

--  As mentioned before, you can also specify an attribute which does not exist yet in the "destination" field in order to create it.

--  The blend slider will blend between the original value, and whatever value you come up with after the operation.  Keep in mind that you can use an attribute to determine how much of the blend occurs.

--  You have the option of scaling, adding, and applying a process to the source attribute before the "Operation" takes place.

Operations:

--  Keep in mind that the "Scale" "Add" and "Process" settings will be applied first before any of these "Operations" take place.

--  Copy = The source value is the candidate, the destination attribute value is ignored.
--  Add = The sum of the source value and the destination attribute.
--  Subtract =  Subtract the source value from the destination attribute.
--  Multiply =  Multiply the source value with the destination attribute.
--  Divide =  Divide the destination attribute by the source value.
--  Maximum =  Use the largest of the destination attribute and the source value.
--  Minimum =  Use the smallest of the destination attribute and the source value.

Processes:

--  None = No processing is performed.
--  Reciprocal (1/x) = The value is inverted. 1/0 is set to 0.
--  Clamp 0-1 = The value is clamped into the range 0-1.  Note that this will clip values that are below or above 1 or 0.  
--  Complement and Clamp 0-1 = The value is complemented (subtracted from one) and then clamped to the range 0-1.
--  Threshold 0.5 = If the value is strictly greater than 0.5, it is set to 1. If less or equal, it is set to 0. This can be used to do a hard-mix.

Post Processing:

--  After the Operation takes place, you can further affect the resulting value by adjusting these parameters

Overall Scale:

--  Multiplies this value against the result of the operation.

Threshold:

--  According to the docs, "If the attribute exceeds this threshold value, replace it with the maximum clamp value. If it is below or equal to it, replace with the minimum clamp value. For vector attributes, this is done per component."

Clamp Max/Min:

--  This will prevent values from going above and/or below these values

Create Missing Destination:

--  If the destination attribute doesn’t exist, try to create it by matching the first of the source attributes.

Create Missing Destination as Scalar:

--  If the destination attribute does not exist always create a float attribute. This has the effect of making an implicit length computation for all source vector attributes.

Delete Source Attributes:

--  Remove all the source attributes after processing, unless one of them is also the destination attribute. This is useful for cleaning up temporary attributes that were created to assist in computation.

Error on Missing Attributes:

--  If specified attributes other than the destination are missing, raise an error. If erroring is disabled; missing source attributes will cause that combination to be skipped as if it were disabled. Missing blend attributes will be treated as if the blend attribute was 1.0 everywhere.