This is the Option symbol, which you’ll see quite a lot of as you use Mac OS X.
Depending on the age of your Mac or your keyboard, the key itself might have all sorts of different things on it. It might just have the option symbol. It might have the word “Alt” on it, or the word “Option”; or both; or some combination of one of those words and the symbol.
This can make things confusing for newcomers. To make it easier, remember that Option = Alt = ⌥, and that the key you need is adjacent to the Command key.
Just as the Mac Command key is similar to the Windows Control key, so the Mac Option key is similar to the Windows Alt key, or the AltGr key. Hold it down while pressing other keys, to make them do different things. It’s a modifier.

