On pg 195 of the PHB it states:
When a creature can't see you, you have advantage on attack rolls against it.
So, does this mean I have to successfully hide from the creature, or if it was fighting a creature in front of it and I walk around back, do I get that advantage?
Best Answer
Normally no. But in the right circumstances, yes.
In most cases, a creature is assumed to be moving around in its space on the battlefield, not fixedly focused forward on one opponent (that's suicidal, for exactly the reason you present). So, normally, no, you can't just walk around an opponent and get advantage on them: they see you, see where you went, and are keeping an eye on you.
The exceptions happen in, well, exceptional circumstances, as judged by the DM's good sense. To demonstrate, I have to make something up, so let's have an example.
There, in that moment, you're not hidden. You didn't have to make a hide check to get there, but notice how you're out of the target's sight. This is an example of not being seen.
DM's judgement
The DM's judgement is very important in 5e, for exactly this reason. Most of the time it will be obvious: you're not hidden, and the target can see you just fine. In rare circumstances — often, circumstances that you've put some work into creating — you'll be unseen despite not being properly hidden, and then you'll have advantage if you need to put a knife in their kidney without actually hiding.