A publisher is a company that can help a developer handle things like marketing, support and other things. Basically the same as a book publisher not actually writing books, which is what authors do. Sometimes, the publisher and the developer is the same company, sometimes there is only the developer, and sometimes they are two different companies.

A porting house is a company that specializes in porting games across platforms. Lets say you've built a PC game with Godot, and now you wanna launch it on PlayStation. Instead of doing the porting yourself, you can outsource this to a company who would do the port for you.