If you hold something up you raise it into the air and keep it there.
He’s holding up his bike.

Something that is supporting a structure, such as a post, is holding it up.
You can’t knock down that wall because it’s holding up the roof.

Hold up can mean to delay, especially the movement of traffic.
An accident on the freeway held us up.

To hold up a bank or a person is to rob them by using a gun. Such robberies are called hold-ups.
He was so desperate he tried to hold up a bank.