Can't understand example using continuations

Question

I'm reading the r6rs Scheme report and am confused by the explanation of continuations (I find it to be too dense and lacking of examples for a beginner).

What is this code doing and how does it evaluate to 4? Why does call/cc want an argument that's a function of one argument? How is call/cc's argument used?

(+ 1 (call-with-current-continuation
       (lambda (escape)
         (+ 2 (escape 3)))))
          =⇒ 4

This example is from section 1.11 - Continuations.