Microchip's (and therefore everyone else) use of the word 'fuse' is pretty grim.
It gives visions of a little bit of wire that will pop.
It's just a register on the PIC.
If it is set then a 'dip' in the power will cause a reset (restart) of the PIC.
The threshold of the power dip can be adjusted.
e.g. if it dipped below 2.5V supply then do a restart.
That's just an example. Supply threshold Voltage levels can be set during programming.
And all these adjustments vary between PIC types.
It's done because power 'dips' (i.e. 'brownouts' as opposed to complete power-off 'blackouts') can upset registers and could mugger up your code - even though your code may still run.
Cliches: Better Safe than Sorry and Belt and Braces. So, if it might be iffy then best reset and start again.
The little circuit in the PIC that detects brownouts can, in many cases, be switched off to save power.
This can be important in very low power circuits but then it is up to the user to keep an eye on the supply level. How you do that is the subject of another thread....
If you are bored one day then dig out the actual PIC Data Sheet from Microchip and have a read.
If you are a total newbie then it may confuse more than clarify at first.
As to the nitty-gritty of setting it then that's for another day.
