This two character sequence #! is called shebang. It is used in shell scripts to determine which interpreter should be used to run the script.

I personally like to remember #! as hash (#) and bang (!).

According to Wikipedia:

The name shebang for the distinctive two characters may have come from an inexact contraction of SHArp bang or haSH bang, referring to the two typical Unix names for them. Another theory on the sh in shebang is that it is from the default shell sh, usually invoked with shebang.