how do you burn fuses :
you burn fuses if you update to Official Nintendo Firmware by connecting your swith to the internet. (burning fuses prevents you from downgrading )
how not to burn fuses: Don't update your nintendo swicth by going online (nintendo servers)
in order to upgrade to a recent firmware AND not burn fuses you need to update via homebrew applications. you need 2 files First downloading the firmware from here https://ayakalounge.me/Switch/FWlist
and then second file use this program to update https://switchtools.sshnuke.net/
PS: example of a burn fuse., if you are on firmware 5.x.x and you update to official firmware 8.1.0. you cant ever use any lower firmware other than 8.1.0.
But if you stay in 5.x.x and you update using the guide above, you can use any firmware from 5.x.x, 6.x.x, 7.x.x and 8.x.x
nintendo tries to patch exploits in every new firmware release so the lower you stay on the better