It's the scripting language used by wc3 triggers. It allows many things not possible (or very difficult) with ordinary triggers. Go to Edit>Convert to custom text in the Trigger editor to see what Jass code looks like.
function foobar takes string me returns string
local string str1 = "// Is this a comment or a string?"
local string str2 = me
// local string = "String in a comment! ^^"
return "gdfgdfg\"fsdfsdfsd"
endfunction