is it like infinity/acts like it or? someone explain too me how they actually work please and ty

Google

It sets the resists of a chosen element immune monsters to 95% - then whatever gear or skills you use that lower it further will allow you to actually kill a (previously) immune monster. Comes with a price of your own resists lowered by a ton.

ty is your res lowered by the same number or do all res suffer? havent actually got a sunder charm yet

If it's a cold sunder then it affects cold immune monsters and lowers your own cold res - other monsters or resists are not affected.