Homebrew is a fan term that can be roughly defined as
Any content within a TTRPG game that cannot be found in an official rulebook.
When you customize rules or use any non-official content, you're adding Homebrew to your game!
Some gaming groups will use Homebrew, and some won't. When joining a new group, it's a good idea to ask if they use homebrew. If you're starting a new group, it's also a good idea to check in with everyone and ask what their preferences are with homebrew rules.
You can use homebrew to create your own modify rules, create your own monsters, or even create your own campaign story! With homebrew, the sky's the limit, and your creativity abounds.
Below are some tools that you can use to make your homebrew content. Like many TTRPG resources, they are branded for D&D but can easily be adjusted for the system you want.