What does "var FOO = FOO || {}" (assign a variable or an empty object to that variable) mean in Javascript?

Looking at an online source code I came across this at the top of several source files.

var FOO = FOO || {};
FOO.Bar = …;

But I have no idea what || {} does.

I know {} is equal to new Object() and I think the || is for something like "if it already exists use its value else use the new object.

Why would I see this at the top of a source file?