Transition diagram for following token number
digits(.digits)?(E[+ -]?digits)? is a regular expression for the set of strings that the pictured automaton accepts. That is, all of the strings accepted by the automaton will match this pattern. If you're unfamiliar with regular expression syntax, you can check out this explanation from Wikipedia. 

In particular, if you see some pattern followed by the ? operator, it means that pattern is optional. So something like digits(.digits)? will match strings of the form digits.digits or just digits because the last (.digits) part is optional. 

I would guess that digits is a placeholder for a series of numeric digits (0-9). So something like digits(.digits)? would match strings like "33.5", "0.8", "1.892", "7", etc.