Parameters are bits of data you send to command-line apps, functions (if it were a programming language), or in the case of wikis, templates. Since the first case doesn't really apply to FANDOM wikis, we'll talk mostly about templates, but a little about functions (since FANDOM wikis use Lua, the programming language).
If you use templates in the form of {{mytemplate|...}} you have probably seen parameters as the parts that show up after the template name. So an parameters would be like this: {{mytemplate|paramvalue1|namedparam1=somevalue}} There are lots of variations for how you can use parameters in templates, but so much depends on how the template decides to use them and which types of parameters, the wide variety is unfortunately out of the scope of this article.
Lua also uses parameters, but in a different way. Most Lua modules (they live in pages starting with Module:) take parameters passed to them by templates via something you might see called {{#invoke:...}}. Lua also uses parameters when declaring and calling functions, but it looks more like this function myFunction( param0, param1 ) when declaring a function or this myFunction( passparam0, passparam1 ) when calling a function. As with templates, the workings of parameters in Lua is out of the scope of this article.