The scheme used to encode characters in the file. UTF-8 is the most likely choice.

There are lots of different characters in the world: Latin characters (as used in the English alphabet), Arabic, Chinese and many more besides. An encoding scheme is the way in which these characters are numbered and stored in a file. Unfortunately, historically every sub-region had its own encoding, and if you get the encoding mismatched, you will get weird characters. Recently, UTF-8 has become something of a common standard, but many other encoding schemes are still in use. Try UTF-8 first, but try then others if that doesn't work.