What is the “fifth part” mentioned in Leviticus?

The “fifth part” is mentioned in verses such as Lev 5:16; 6:5; 19:25; 22:14; 27:13,15,19,27,31 in the King James Version. It refers to a 20% fine or tax. For example, if someone steals something, he must pay back what is stolen plus a fifth or 20% (see Leviticus 6:1-5). It is clearer in modern versions.