Think of it like this:
If you are invisible, you are heavily obscured from enemies. This means they have disadvantage on attack rolls against you. They know approximately where you are, so they can try to attack, but will have a difficult time hitting.
If you take the Hide action, you try to be silent as well, so you don't give away your location at all. Enemies have to guess where you are (if they cannot beat your Stealth check). So they could attack at random, or try to deduce where you'd go, but it's still guessing. They could hit a space where you simply aren't. And if they do hit the right space, where you are, they still have disadvantage on ther oll.
So if you are a Wizard who casts Hide, the first turn enemies will have disadvantage on attack rolls against you. On your next turn, you can try to Hide if you want to completely sneak away.
I'd say that the Ranger ability you're qouting means this:
As with most things in D&D, there are too many possible scenarios to specify rules for everything, so the DM will definitely have to make judgement calls every now and then.