The pen tool in Adobe After affects works in the same fashion as the same tool in illustrator working with anchor points to create a line of motion. The Convex Vertex pen converts curves into set points.
Using the Keyframe Interpolation window you are given the options editing and changing what happens before and after a Keyframe. When working with motion it converts movement into keyframes.
Before Hold Interpolation
Note to self: The number of dots on a line within after effects represent Frames per second, the more dots on a line the longer something with take to move the fewer dots the quicker it will take.
After Hold Interpolation
When working between Illustrator and After effects you can use a shape or path from illustrator copied to a keyframe enabled layer in after effects to create a path of motion.
To create a solid layer that isn't just a square as in the previous screen shots a mask is used and shapes from illustrator can be copied across to use. Mask Options;