In the modifier section of the feats? there's a Modifier type called Favored Enemy. And the Sub type are the enemy categories that you can choose from the Ranger? But what exactly does this tdo?

It designates creatures of the selected type as the character’s Favored Enemy.

Favored Enemy

Beginning at 1st level, you have significant experience studying, tracking, hunting, and even talking to a certain type of enemy.

Choose a type of favored enemy: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, monstrosities, oozes, plants, or undead. Alternatively, you can select two races of humanoid (such as gnolls and orcs) as favored enemies.

You have advantage on Wisdom (Survival) checks to track your favored enemies, as well as on Intelligence checks to recall information about them.

When you gain this feature, you also learn one language of your choice that is spoken by your favored enemies, if they speak one at all.

You choose one additional favored enemy, as well as an associated language, at 6th and 14th level. As you gain levels, your choices should reflect the types of monsters you have encountered on your adventures.

Mechanically it doesn’t “do” anything at the moment. The benefits from Favored Enemy only apply “some of the time” since not every Wis (Survival) or Int check will be against your FE. DDB can’t do “temporary”/“toggleable”/“modal” stuff like that yet.