Pattern

Pattern[symbpatt] or symb : patt
assigns the name symb to the pattern patt.
symb_head
is equivalent to symb : _head (accordingly with __ and ___).
symb : patt : default
is a pattern with name symb and default value default, equivalent to Optional[patt : symbdefault].

Pattern has attribute HoldFirst, so it does not evaluate its name:

Nested Pattern assign multiple names to the same pattern. Still, the last parameter is the default value.

This is equivalent to:

FullForm: