To use the lattice node, you'll need to plug geometry into all three inputs of the node.  The first input will receive the geometry that you're trying to deform.  The second is asking for the control object (usually a bounding box) at its rest position.  It's "rest" position is the position of the points at their default locations.  The third input is asking for the same object you put into the second input.  This time, however, you want to move those points around to deform the control object.  With all that data in place, you'll have what you need to deform your object.

Group:

--  This is asking you which points are allowed to be affected by the lattice node.  For more information about groups, please visit the Group Node Bible entry.

Lattice:

--  Use this mode if you intend on deforming the object using a bounding box.

Divisions:

--  Set these divisions equal to the divisions that are present on your control bounding box.

Interpolation:

--  This will slightly change how the deformation is interpreted from the deformed bounding box to your model.  In practice, this doesn't make a huge difference, but may be useful if you'd like a slightly different result from your lattice operation.

Points:

--  Unlike the Lattice tab, this will allow you to use any kind of control mesh you'd like to deform the object.  This works by turning each point of the deforming mesh into a metaball weight that influences the geometry.  To visualize the influence of each point, you can turn on the "Visualize Point-Specific Radii"

Rest Group:

--  By specifying points here, it will help optimize this mode by only allowing certain points of the control geometry to affect the mesh.  

Kernel Function:

--  These Kernel Functions can control the falloff that's used to influence the geometry.  For more info on Kernel Functions, visit the Attribute Transfer Node Bible entry.

Radius:

--  This controls the radius of the points as they influence the underlying mesh.  

Normalize Threshold:

--  According to the docs, "Defines the transition sharpness of the deformed surface geometry across the capture region boundary. If the threshold is zero, the transition is steep. If the threshold is large, the deformed data source geometry is smooth. Technically, the threshold defines the value after which the weights are normalized. If a total weight of all capture regions for a given point is greater than the threshold, then the weights for this point are normalized; otherwise they are not."

Visualize Point-Specific Radii:

--  This allows you to visualize the weight of each point along the deforming mesh.