Why do I get bubbles in polyurethane, and do I get rid of them?

I always dilute mine (which is oil based) with minerals spirits (2 parts varnish to 1 or 2 parts urethane varnish) and apply it by wiping with an old cotton sock. It takes more coats but they can be applied more frequently. Limited clean-up. I wear a disposable glove and store the sock in the polyurethane between coats.

Don't stir or shake

Could be the temperature of room too cold?

You might have stirred or shaken the product or you might have brushed it on too quickly. You could pop the bubbles with a straight pin or let what you have already brushed on dry then lightly sand. Be sure to clean off residue after sanding then try again for a second coat - go slow

Thank you all. No temperature warm, outside in the California sun.

Hope you didn't shake the can. Gentle stirring is all that's needed. Using the wrong type of brush. I like to use a foam brush. Cleaning the brush on the container edge. You can burst any bubbles while it's still wet. Use a propane torch and pass the heat over the poly, bubbles will burst.