The word translated as "understand" means "to bring together." It means "understand" in the same sense that we say "put it together" to mean "understand." This tells us that Christ's idea of "understanding" was more like putting together pieces of a puzzle, not a passive process. It could be a command, but it could also be a simple statement.
Jesus uses a couple of different Greek word that can be translated as "hear." This particular one means not just hearing with the ear, but listening so you can understand. It can also be translated as "understand" though the KJV never does that.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.