The easiest thing to do is to look for an example project with a .cfg file and use that.
The linker config file is used during the assembly process to control the layout of your ROM file. Basically just what data goes where. There are a lot of different
mappers used for the NES, and they each have a different set of needs, in this respect.
Probably most examples would be using a config suitable for the NROM (simplest possible) mapper. That's what should usually be recommended as a starting point.
As far as what a
linker is, the assembler used has two phases. In the assembly phase, it turns your assembly code into "object" files, which are basically machine code blobs, a direct translation of the stuff in your assembly files. The linker phase combines those object blobs into your finished ROM ("linking" them all together).
If you want to dig deeper into it, there's documentation about linker config files here:
http://cc65.github.io/doc/ld65.html#s5