To applaud is to show approval, encouragement, or appreciation. You can applaud by clapping your hands at a performance, or you can applaud with just your mouth, as when you tell someone "I applaud your decision to start exercising."
The verb applaud goes back to the Latin verb applaudere, "to clap," and it came into English use in the 15th century. Part of the word applaud is laud, which means "praise," and laud is close in spelling to loud. When you applaud, you give loud praise by clapping you hands together. If you really like what someone is doing and want to pat them on the back for it or show encouragement, you might just say "I applaud your efforts."