What does this mean?

.NET
1 
 Dim minutes As Double = 0.0!

what does the ! at the end mean?

is it a type character identifier?

2

! = 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.

3

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