To quote the intro to the Godot documentation:
Godot is completely free and open source under the very permissive MIT license. No strings attached, no royalties, nothing. The users' games are theirs, down to the last line of engine code. Godot's development is fully independent and community-driven, empowering users to help shape their engine to match their expectations. It is supported by the Software Freedom Conservancy not-for-profit.
Open source means that it is available for you to use for free for whatever projects you like. You do not need permission nor do you need to pay anything to use it.