What does "with" do in JavaScript?

the with statement is pure syntactical sugar, but it also can cause some nasty bugs.

See with Statement Considered Harmful for clarification:

If you can't read a program and be confident that you know what it is going to do, you can’t have confidence that it is going to work correctly. For this reason, the with statement should be avoided.

In that with block you dont have to type:

sObj.options[selectedIndex].value

but you can just use:

options[selectedIndex].value

It adds to the scope of the statements contained in the block:

return sObj.options[selectedIndex].value;

can become:

with (sObj)
    return options[selectedIndex].value;

In your case, it doens't do a whole lot...but consider the following:

var a, x, y;
var r = 10;
a = Math.PI * r * r;
x = r * Math.cos(PI);
y = r * Math.sin(PI /2);

Becomes:

var a, x, y;
var r = 10;
with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}

...saves a couple of keystrokes. The Mozilla documentation actually does a pretty good job of explaining things in a little more detail (along with pros and cons of using it):

with - Mozilla Developer Center