Fabric - a set of libraries that allow third-party developers to modify the game, in other words, run mods in minecraft 1.14 and higher. Fabrik is still in the early stages of development.
How to install Fabric
1. Install Java
2. In the launcher, start and close the minecraft of the desired version
3. Download the file from the link below
4. Run the file
(If the file opens as an archive, then right-click on it and select "Open With" -> "Java")
5. Select the desired version of minecraft and click the "Install" button
6. Restart the launcher
7. Install Fabric API
8. Done