It takes a value in one range and returns it in another.  If you have data on a scale that goes from -5 -> 5 but you need a value in the range 0 -> 255 (to use as a colour for instance) then you can use map to do the maths of that conversion for you (the maths isn't difficult but Processing saves you the trouble).

Let's say I wanted to draw a circle inside a box and control the circle position using the mouse over the whole window.  If I just give the mouse position to ellipse() then that'll draw outside the box so instead I can use map():

1. void setup() {
2.   size(640, 480);
3.   ellipseMode(CENTER);
4. }
5. 
   
6. void draw() {
7.   background(150);
8. 
   
9.   // our mouseX can go from 0 -> 640 and our
10.   // rectangle lies between 50 and 250
11.   float x = map(mouseX, 0, 640, 50, 250);
12.  
13.   // our mouseX can go from 0 -> 480 and our
14.   // rectangle lies between 200 and 400
15.   float y = map(mouseY, 0, 480, 200, 400);
16.  
17.   // draw the rectangle
18.   rect(50, 200, 200, 200);
19.  
20.   // draw the ellipse using the position we
21.   // worked out with map()
22.   ellipse(x, y, 10, 10);
23. }

Thanks it was useful