This data pack multiplies the value of Experience Orb entities by a configurable amount. This means experience gained by the player, as well as Mending repair rates, are both multiplied.
Note: Version 1 & 2 of this data pack multiplied all experience gained from every source, which sabotaged the /experience command and advancement rewards. This is no longer the case with Version 3+.
By default the data pack multiplies the amount of experience points by 2. If you'd like to change that, you can find the configuration file at "data\exp_multi\functions\config.mcfunction" within the zip file of the data pack.
If you'd like even more sources of experience to multiply, you can get my Extra Experience data pack to increase your gains!
More information about data packs and how to use them can be found on the official Minecraft Wiki.