Let us start with what we know about sheaves, i.e. the "1-level". A sheaf on a (Grothendieck) site $\mathcal{C}$ is a contravariant functor $F : \mathcal{C}^\text{op} \to \textbf{Set}$ such that for any cover $\{ X\ \to Y\}$ , the diagram
$$F(Y) \to F(X) \stackrel{\longrightarrow}{\longrightarrow} F (X \times_Y X)$$
is an equalizer in the category of sets. For the sake of exposition, I will only consider the case where the cover consists of a single element.

Now suppose we want to move to the "2-level" and talk about stacks. Then we need to throw in the cocycle condition, so we can **naively define** a stack $F$ to be a contravariant functor $\mathcal{C}^{\text{op}} \to \textbf{Set}$ such that

$$F(Y) \to F(X) \stackrel{\longrightarrow}{\longrightarrow} F (X \times_Y X)\stackrel{\stackrel{\longrightarrow}{\longrightarrow}}{\longrightarrow} F(X \times_Y X \times_Y X)$$

is an equalizer in the category of sets. The problem now is the following:

If $F$ is some kind of moduli stack, e.g. $F = \mathcal{M}_{1,1}$ then to make $F$ set valued often involves quotiening out isomorphisms. However, this is very bad as the presence of quadratic twists of elliptic curves means $F$ is not injective.

So now what do we do? Well, we can try to not quotient out isomorphisms, and think of $F$ as a **groupoid** valued functor. But now we have a new problem:

If $F$ is valued in groupoids, what does it mean to say that $F(Y) \to F(X)$ is "injective"?

The solution is the following. Let's go back to situation where $F$ is a plain old sheaf, and let us think of the set $F(X)$ as a category where the only arrow $x \to x'$ is when $x = x'$, otherwise $\operatorname{Hom}(x,x') = \emptyset$. Then now to say that $F(Y) \to F(X)$ is injective is **exactly equivalent** to the statement that the functor $F(Y) \to F(X)$ is **fully faithful**.

The upshot is that to make the right definition (of a prestack), we now know that:

- $F(X)$ should be a groupoid.
- $F(Y) \to F(X)$ should be fully faithful.

We're not there yet, and we need one last modification (at least for $F$ to be a prestack. We need to replace $F(X)$ with $F(X \to Y)$, namely the **category of covering data**. The objects of this category are pairs $(y, \phi)$ where $y \in F(Y)$ and $\phi : \text{pr}_1^\ast y \to \text{pr}_2^\ast y$ is an isomorphism. A morphism of covering data $ (y, \phi) \to (y', \phi')$ is a map $f : y\to y'$ such that an appropriate diagram commutes (see chapter 8 of the book "Neron Models" by BLR for the exact definition). We can now define:

A groupoid valued functor $F$ (or pseudofunctor in Vistoli's language) is a **prestack** if the natural pullback functor $F(Y) \to F(X \to Y)$ is fully faithful.

If you unravel what the morphisms are in the category $F(X \to Y)$, you will see this is exactly the condition that the **set-valued** functor $\underline{\operatorname{Isom}}$ is a plain old sheaf!

So we can finally get to your question. In my view, a stack **is** a generalized sheaf if you replace sets with groupoids, and if you introduce the category of covering data.

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