Hotswap
From Forge Community Wiki
What even is this?
Hot-swapping is a feature letting you to see code/resource changes in-game without restarting it. This article is divided into 2 sections: Basic hot-swapping and schema change hot-swapping, as latter one requires more setup.
Basic hot-swapping
Here, you would be able to swap out resource pack/data pack of your mod without restarting the game. You can also swap out code if only method's internal has been changed. You would still need to restart game if you added/removed/renamed method/variable/class.