This data pack causes Slimes to drink milk from milk buckets dropped by the player, which causes the slime to increase in size. Fed Slimes become persistent, and the Slimes that spawn from them when they die are also persistent, which allows the player to farm Slimes in a similar fashion to passive mobs. Magma Cubes can also be grown in the same way, but they prefer to drink Mushroom Stew.
By default Slimes and magma cubes can grow to a maximum size of 3 (the largest size they naturally spawn as), and there is a 5 minute cooldown period after feeding them, during which they cannot grow again (the same time as breeding passive mobs). If you'd like to change that, you can find the configuration file at "data\slime_b\functions\config.mcfunction" within the zip file of the data pack.
More information about data packs and how to use them can be found on the official Minecraft Wiki.