There can be several reasons why there is a rubber band on your mailbox which can include you have a large package or that you have mail or a package waiting to be picked up.
Sometimes the mail carrier will place a rubber band on your mailbox to hold the lid shut if a package is too large or if the door of the mailbox won't stay shut.
And sometimes I've had packages that wouldn't fit in my mailbox hung outside of and attached to the mailbox with a rubber band.
If it's not the mail carrier adding the rubber band to your mailbox then it could be someone doing a prank.
However mailmen and postmen and mail carriers do carry rubber bands to hold your mailbox door shut or partially shut if damaged or if there's a danger of a package falling out.
Some people will also use a rubber band on a mailbox to signal to the postal carrier that there is mail in the box that the customer wants picked up and delivered to it's destination.