Julio - I guess you're happy now, but a "diagram of shape \\(\mathcal{A}\\) in the category \\(\mathcal{B}\\)" is just a functor \\(F : \mathcal{A} \to \mathcal{B}\\). (I'm deliberately using completely different letters, because you should never get attached to particular letters.)

So, for example, there's a category \\(\mathcal{A}\\) called the **square**:

and we can look at a diagram of this shape in any category \\(\mathcal{B}\\). Very roughly speaking, it's like a picture of shape \\(\mathcal{A}\\) drawn on the canvas \\(\mathcal{B}\\). This is a very important and beautiful idea.

So, for example, there's a category \\(\mathcal{A}\\) called the **square**:

and we can look at a diagram of this shape in any category \\(\mathcal{B}\\). Very roughly speaking, it's like a picture of shape \\(\mathcal{A}\\) drawn on the canvas \\(\mathcal{B}\\). This is a very important and beautiful idea.