If they're talking about resists over 100 they are most likely referring to overstacking resists. You put on, for example, enough gear to be at 200 resist lightning in hell difficulty, but the game caps you down to 75. However, when you are effected by conviction, it subtracts form the 200 FIRST and then looks at the cap, so 200-125 = 75, so you're still at the resist cap, even through conviction.
You can also increase max resists as sunset says, but I don't think that's what "people" are talking about, certainly not if they're talking about being over 100.
I don't agree with sunset that increasing max resists is useless. I've used guardian angel to good effect against ubers. Even at 75 resists under conviction the elemental damage can be quite substantial there. With guardian angel last aldder I was able to reliably not die, to spite not having CtA for battle orders (early on that is, obviously I got a CtA eventually). I would certainly agree that it is useless if you are not able to hit resist cap under conviction though, that being the case you're better off with smoke, or anything really that has resists.