Serial
Serials are used by MTA and server administrators to reliably identify a PC that a player is using. Serials are 32 characters long and contain letters and numbers.
Serials are most used for banning players, because a computer with a dynamic IP can change IP every now and then, making IP bans ineffective. MTA wants to give server owners a more reliable way to ban players and keep them out, which is why serials were invented.
Notes
Serials are the most accurate form of identifying players that MTA has. However, you shouldn't rely on them entirely: players may change their serial, although this is very difficult to do due to MTA AC protections (serials aren't only based on hardware, but more). Besides, a player can simply use another computer with a different serial in order to fool your scripts or evade server bans.