Here's why:
* Pónganselas is a conjugation of the verb "poner" (to put) in the informal plural imperative form.
* se is the reflexive pronoun in this case.
* las refers to a feminine plural object (likely clothes).
Breakdown:
* Pónganse = Put yourselves (reflexive imperative)
* las = them (feminine plural object)
So, the complete meaning is "Put them on yourselves."