It's not a function. You declare and initialize a variable named "map". It creates an array of 32 cells. An array index start to 0. eg: if you save the string "de_dust2" in this variable, map will contain the string like that :
0 : d
1 : e
2 : _
3 : d
4 : u
5 : s
6 : t
7 : 2
etc..