Easing in is where you have 2 pieces of fabric that you want to join, but one is longer than the other. If it's 2 patches that you are joining, and the difference in lengths is small, you pin the 2 edges, sew with the longer piece is on the bottom (sew with baggy bottom) and tug a bit on the top piece. The feed dogs will ease the fullness in.
On longer pieces, like when joining a border to a top, you need to do a bit more. Locate the center of the top and the center of the border & pin these together. Next pin the 2 ends. Now locate the centers between the pins on both the top and border & pin those together. Continue finding the center spots between the pins and joining. This prevents all the excess fabric from the longer piece winding up as pleats.