It’s for the air conditioning compressor and as said it’s a stretch/single use belt. Easiest way to get it off is to cut it… as said there is a special tool to refit the new one but it is possible to do it by hand - load it fully onto the idler pulley and as best you can on the ac pulley then turn the idler to pull it fully on that one. Also as said above you can do it by turning the engine on the crankshaft nut but you must loosely install the stretch belt an the idler only and then refit the main belt over it to be able to do that.
This chap uses a tie wrap to help but it can be done without…..
https://www.youtube.com/shorts/v3uLWgoExzU
But of course it is always best and easier to have the correct tool - copies of the Volvo tool and other variations that will work are very cheap on eBay (some variations at >£10) too many failed attempts doing it without the tool may mean it’s overstretched and slips.