Basically, this three-act play takes place in a very small town in the early 1900s. The main characters are basically two families and a few other citizens of Grover's Corners, the city it takes place in.
Each act represented a different stage in life, so the last stage, obviously, was the best part. The overall message that was revealed in the last act was quite inspirational.
This play is a classic, if you didn't know, known as "THE great American play."