when you layer the garden with trees for a canopy, shrubs under them and then smaller plants such as ferns and hosta you are underplanting. Nature does this in our forests. Each layer provides cover and shelter for the one below. We can mimic this in the garden when we understand the needs of each plant.
Also: Not planting enough of something. I find my underplanting of tomatoes this year to be something to correct in next years garden and that my overplanting of zuchinni has all the neighbors hiding for fear I will try to give them some more!