Compose is also a name of the key that's commonly found on many terminal keyboards.
What it does is roughly same everywhere: When you press Compose, and then two other keys, something similiar to combination of these two glyphs appear on the screen.
For example, Compose-/-O will give Ø, Compose-"-O will give Ö and Compose-O-C will give ©.
In X Windowing System this key is better known as MultiKey, and you can use xmodmap to map it to any key. Also, on my case at least, telling XFree86 that I have a 105-key Finnish keyboard turned right Windows key automagically into MultiKey.