What does this mean?

 Dim minutes As Double = 0.0!

what does the ! at the end mean?

is it a type character identifier?

! = Single; example: Dim Q!

Literals can also use the identifier type characters (%, &, @, !, #, $), as can variables, constants, and expressions. However, the literal type characters (S, I, L, D, F, R, C) can be used only with literals.

Ya you are right.! means single.
0.0! means 0.0 is a single type number.Float is of two types single or double.