Sometimes you'll want to use a function from another mod, or use some item provided by another mod in a craft recipe. To make sure that the function/item is available you need your mod to depend on the other mod.
When this video was recorded we used a depends.txt file, however now this information should be provided in a mod.conf file.
depends = mod_you_need
optional_depends = mod_you_can_use_but_don't_have_to_have