Some desktop environments set a default compose key, but you might have to set one manually. Common choices are the menu key or the right alt key if you don’t use it much.
Mostly it just defines a set of pretty standard and sensible combinations to add accents or other modifiers to existing characters, but there’s quite a bit you can do with it.
If you don’t use a DE, it looks like there are ways to enable it in window managers as well. You’ll have to look up specific instructions for yours.