the first is to create a value for interpolate, that you can retrieve from expressions.
The second is to interpolate object positions,rotations,and dimensions automatically over a given time. like being able to order an object to a location. there are also triggered conditions for when interpolation completes, and actions to pause and resume interpolation. using the automated interpolated value actions and expressions, you could automate anything like pv's, opacity, and color.
also, the interpolation allows you to set a speed curve control point anything from 0 to 1, 0 is ease in, 0.5 is linear, and 1 is ease out.
hope it comes in handy, have fun!!!