Something is capped if it has something on top of it. Birds with a colored patch on top of their heads are often described as capped. Mountains covered with snow are called snow-capped. Something that has been closed with a lid on top has been capped. The well in the picture has been capped.
Clapped is the past tense of clap, for example "they clapped after the performance". See also: applause.
You are trapped when you are stuck in a place you cannot get out of. The possum in the picture is caught in a trap.