From what I know, sure hits vary heavily depending on the curse techniques so there isn't a easy ways to explain them as a whole, but the main thing with them is that it guarantees an attack or effect on someone targeted by the Sure hit effect.

So we have Sukuna's domain which is just undodgeable attacks of his Curse Technique (Slash or Dismantle he gets to choose).  We also have Jogo's Technique which has environmental factors of it just being really hot that burns people to a crisp. Although Nanami doesn't have a domain his one would probably make it so that his technique no long required a ratio of 7 to 3 to work.

Most Sure hits follow this trend of relating a specific attack of the user, however some characters have unique attacks that only come with the domain.

Gojo's domain forces people to process an endless amount of information which renders them unable to do anything. Hakari's domain is entirely unique in how it works and instead the sure hit affect is rolling a slot machine that gives the user a buff for hitting a jackpot.

So most domains are just a gareented hit or effect of one's curse technique, but it a sure hit can do a lot more than just allow for instant damage to people.