When two pieces of fabric are not exactly the same length, "easing" means sewing so that the extra fabric in the longer piece is distributed over the shorter length. When sewing a row of blocks together, for example, some blocks may be slightly larger than other blocks. You will see this as soon as you pin the rows together at the block intersections. Typically placing the longer piece underneath and exerting a little "pull" on the top piece results in the longer piece being eased in. This is because the feed dogs typically feed the fabric more than the presser foot; the presser foot "pushes" fabric ahead of it.
Clear as mud?
It helps if someone can show you.