See here:

https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers

In summary, the Internet Assigned Numbers Authority officially assigns certain ports for certain services, but in other cases it's just convention, and either way you are completely free to ignore it if you want. For example I'm a web developer and so I routinely run web servers on my laptop for local work, if you're running multiple for different projects you might stick them on 8080, 8081, whatever. Also sometimes people use non-standard ports as a form of security by obscurity. (To continue parent analogy somewhat, imagine if every apartment building was supposed to have a bank vault in apartment 762, it would make it kind of easy for thieves to find, so you might stick it in 638, and have an intermediary stationed at 762 to vet anybody who turns up and only forward known bank customers to the correct place)