The Attribute Promote node is used to convert one kind of attribute to another.  In practice, this node gets used all the time because sometimes solvers and other nodes expect attributes to be point attributes instead of primitive (or vice-versa). 

This node is also useful for times when you want to add, average, find the median, or anything of that sort by promoting as a detail attribute.  Detail attributes only store one value per entry (unlike points, prims, or vertices) so we can calculate this value in a variety of ways through the Promotion Method parameter.

This node also allows you to re-name and delete existing attributes.

Original Name:

--  This is asking for the attribute name that you'd like to promote

Original Class:

--  This is asking you what kind of attribute currently exists (is it associated with points? prims? vertices? or detail?)

New Class:

--  This is asking you what you would like to convert the new attribute to.

Piece Attribute:

--  This section is assuming that you have some kind of attribute that's being used to identify various pieces that belong to a mesh and/or multiple meshes.  So, you'll need to use the connectivity SOP to create a unique piece attribute if you don't have one established yet.  Once you have a piece attribute, (usually called @class or @name) you can then use this parameter along with the "Promotion Method" to assign one value to each piece.  To see this in action, watch the video above at 2:58 and notice how the same color will get assigned to each piece of the fractured mesh depending on what you've selected in the promotion method parameter.  In practice, this is useful for controlling attributes in RBD simulations.

Promotion Method:

--  When the attribute promote runs across multiple values for an attribute, how should it calculate the new values when promoting it to a point/prim/vertex/detail attribute?  That's what this parameter is asking you.  This allows you to take the _____ (average, min, max, sum, etc...) of multiple values when the attribute promote encounters them to figure out the resulting values.

Change New Name + New Name:

-- If you'd like to rename the attribute to something else once its promoted, then you can do so by specifying it here.

Delete Original:

--  This parameter is very important to keep track of.  By default, this node will delete the existing attribute and only keep whatever you promote.  So, if you promote a point attribute to prim, then this will delete the original point attribute and leave only the resulting prim attribute.