By default it's bound to find-tag. However, many modes rebind it to something more specific to a particular language.
You can find out what function a key sequence is bound to with C-h k (for describe-key) followed by the key sequence you're interested in. So in this case you would do C-h k M-..
You can also use F1 in place of C-h, so <f1> k M-. should work too.