The term you’re looking for is “masking”. You can use anything from masking tape to commercially made masking. I use the stuff linked below. You don’t have to mask anything, but it can help with a couple of things. If prevents smoke/soot from depositing on the material which can stain and be tough to clean up on some materials. For some projects I like the effect the smoke can give, other times I like a clean look so I mask. The smoke can often be cleaned so I skip masking when I don’t want to deal with peeling a million tiny pieces off. Smoke doesn’t effect things like acrylic so I don’t mask it for that reason, but I may if I need to protect it.
The second thing masking can do is protect the surface from laser burn or “flash back”. Flashback is when the laser heats up the metal tray and it leaves little burns on the back of the material.
Something to keep in mind is masking takes power to burn away and it will effect the outcome. If you mask using narrow tape and overlap it, the overlapped areas will be different then the other areas. There’s lots of other info out there about it if you want to give a quick search as well.
https://www.amazon.com/gp/product/B07DF7TDSF/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
This discussion is good too