To 'have been playing' is the present perfect continuous tense of 'to play'.
Present perfect continuous is used for actions which started in the past and may or may not be finished.
"They play with their friends." (present simple) means that they regularly or generally play with their friends.
"They are playing with their friends." (present continuous) means that at this moment they are playing with their friends.
"They have played with their friends." (present perfect) means that 'playing with their friends' is something that they have done at some point. At this moment, it's something that has been done.
"They have been playing with their friends." (present perfect continuous) means that 'playing with their friends' is something they have been doing at a time that is relevant to now. E.g. as an answer to "What have they been doing today?", or as part of "They've been playing for the last two hours."