“Stick it in the microwave on "defrost" for a few minutes.”

Your husband is going to cook dinner tonight, but he doesn't cook much so he doesn't know what to do. He has a frozen steak that needs to be thawed out. You explain what to do:

Stick it in the microwave on "defrost" for a few minutes.

stick (something) (somewhere)

To "stick" something in a place means to put it there.

"Stick ___ ___" is more casual than "put" and also gives the impression of putting something quickly. For example, if someone is helping you move into a new house, you can tell them where to put a piece of furniture:

Just stick it over there in the corner.

In the example at top, the speaker says to "stick" the steak in the microwave because she's talking to her husband in a casual situation.

the microwave

The microwave is a machine that heats food up really quickly. A more complete and formal name for it is "microwave oven".

When you're cooking or heating something in a microwave, you can use the expression "microwave (something)":

Microwave it for a couple minutes.

on defrost

"Defrost" is one of the settings on a microwave oven. It's a setting that warms up frozen food slowly so that it doesn't get too hot and start to cook. There's also a "defrost" setting on car heating settings.