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.
They are mostly 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.
Their reliability is so high that many players tend to have the same serial for 6 years or longer (in theory unlimited amount of years) unless they use a different PC at some point. This gives serials a wide range of potemtial uses in scripting and anti-abuse purposes and systems. But for vast reasons, please avoid using them in place of user accounts, a practise that will come with a variety of problems (some of which for security reasons we won't mention, and situations like when a player starts using a different PC and loses their account data without ability to transfer it, or somehow ends up changing their serial as per the below paragraph (intended or by accident). So basically, again, do not rely on serials for critical functions on your server.
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.