Here is a list of some of the functions that you can use specifically in server side scripts.
Server-side native functions are provided by the Citizen framework. Refer to the FiveM Native Reference, where you can see syntax per language, a description, and examples for each native.
These natives are usable in all runtimes.
Some functions are exclusive to the scripting runtime you're using, and are not documented in the FiveM Native Reference List. Refer to their docs for more detail.