What the Ranger feature talks about is also mentioned in the Navigate activity from p. 183 in the PHB:
Navigate. The character can try to prevent the group from becoming lost, making a Wisdom (Survival) check when the DM calls for it. (The Dungeon Master’s Guide has rules to determine whether the group gets lost.)
The mentioned rules are in p. 111 from the DMG, under "Becoming Lost" section. I'm not sure I can paste the whole section here (copyright and all), but the relevant parts:
Unless they are following a path, or something like it, adventurers traveling in the wilderness run the risk of becoming lost.
It then describes the test, the DC and all that stuff.
If the Wisdom (Survival) check succeeds, the party
travels in the desired direction without becoming lost.
If the check fails, the party inadvertently travels in the
wrong direction and becomes lost. The party's navigator
can repeat the check after the party spends 1d6 hours
trying to get back on course.
So, essentially, having the Ranger in the party means you don't have to make this check, ever, because you can't get lost (i.e. you always succeed on this check).
As request by the asker, what does count as a destination? - first, it's not a game-defined word, so it just uses the common dictionary meaning.
the place to which someone or something is going or being sent.
Any place that fits this descriptions works. The example given - 'the tree I buried the treasure under' - surely counts as a destination in my interpretation. It's ultimately up to the DM, though.
