To "try on" clothes means to put them on in order to see how they look on you. You "try on" clothes when you are shopping. You can also "try on" your clothes at home when you want to decide what to wear.
You say "try on ___" with a noun or noun phrase:
She's trying on a pair of pants.
When you replace the noun with "this", "these", "it", "them", and so on, you say "try ___ on":
Did you try them on?