If f(x + y) = f(x) + f(y)

and f(1) = 3,

find f(2), f(3), and f(4). Is f(x + y) = f(x) + f(y) for all functions?

I don't understand what f(x + y) = f(x) + f(y) is.

Is it saying that if you plug in 2 numbers (x + y) into the function f, the value returned will be what would be returned by the same function if you plugged in each number separately ( f(x) and f(y) )?

I also don't understand what f(1) = 3 is saying. Is it stating that I'm plugging in 1 for x + y, so that (x + y) = 1, which then gets sent to the function for calculations?

What's more important to me is getting at the source of my confusion, so that I can solve this on my own.