This won't quite be about sheaf cohomology but hopefully it will communicate the basic idea. Suppose $X$ and $Y$ are two spaces, and you want to compute the mapping space (or maybe just $\pi_0$ of the mapping space) $[X, Y]$. For example, maybe $Y = BG$ for some Lie group $G$, so $\pi_0 [X, BG]$ is the set of isomorphism classes of $G$-bundles on $X$, which you'd like to classify. Loosely speaking, whenever you want to compute a complicated hom, there are two things to try:
- Writing $X$ as a colimit of simpler things.
- Writing $Y$ as a limit of simpler things.
Cech cohomology is a strategy for doing the first thing. Given any map $f : U \to X$ (which we want to think of as some sort of cover; for example, $U$ might be the disjoint union of opens making up an open cover of $X$), there is a canonical diagram you can write down that starts
$$\cdots U \times_X U \rightrightarrows U \to X$$
where the next part is $U \times_X U \times_X U$ and there are three arrows, etc. In general these pullbacks need to be homotopy pullbacks. Altogether you get a simplicial space called the Cech nerve of $f$ which, hopefully, is a "resolution" of $X$ in the sense that $X$ is its geometric realization (which you should interpret as a homotopy colimit). This requirement is an "effective descent" condition; see this MO answer for a bit more on this.
If that's true, then because the mapping space functor $[-, Y]$ sends homotopy colimits to homotopy limits (exactly like a hom should), this lets you write $[X, Y]$ as a homotopy limit of spaces of the form $[U \times_X \dots \times_X U, Y]$, or more explicitly as the totalization of a certain cosimplicial space. This might not seem like much of an improvement, but, for example, if $U$ is a "really good cover" (an open cover such that all finite intersections are contractible) then all of the pullbacks $U \times_X \dots \times_X U$ will be discrete (all of their connected components will be contractible); in this case the simplicial space (really a simplicial set) we wrote down is like a "free resolution" of $X$, and we can write $[X, Y]$ as a homotopy limit of spaces of the form $Y^n$.
With some more work (at some point you'd like to replace talking about homotopy limits with talking about ordinary limits and this isn't formal), this story recovers the Cech cocycle description of $G$-bundles when $Y = BG$, and also the Cech description of ordinary cohomology when $Y$ is an Eilenberg-MacLane space. You can also take $X = BG$ for a discrete group $G$ and $f : 1 \to BG$ to be the inclusion of a point, in which case you'll get back the usual description of group cohomology in terms of the bar resolution.