Not sure if anyone's explained this in depth, but here we go:
Humanity
Humanity is pretty damn important lore wise, but also pretty helpful in game. Basically, it's both a heal item, as well as the 'turn multi-player on.' You get Humanity from one of several ways:
- Killing certain enemies, usually 'human'/un-dead, though some natural monsters as well, have a random chance of giving you a Humanity point (this instantly goes to the bar at the top left of the screen, but be careful, since that Humanity can be lost if you die and don't recover your dropped souls).
- Finding them as items (these item versions are not lost upon death, and hence more valuable for it), usually all over the place, though the most common is from bodies and victories over bosses.
- And (the real clincher) either from NPC's or other player's, namely from their DEAD BODIES.
Their function boils down to: giving you one nearly always full heal (item version only, though, because of their value, you'd be better off using the etus flask until you really get into the game), and making you "Human." Or as human as a wayward soul can be, at any rate.
Being "Human" does two things: allows you see 'summon signs,' and hence able to call other players into your world to help you, once you have the proper item to do so*.
And allows you to be 'invaded' by Dark Phantoms. These guys are basically there to make your life more difficult: while some act with honour, and will at least bow to you before attacking, the majority just want to kill you, since doing so gives them a free humanity (note, that you can do this too, once you find the right items). Hence, it's like risk and reward: being human can make the game much easier, since two lost and doomed souls are better then one, but can likewise make it much more difficult, since invaders are often more willing to see you dead then the monsters you fight.
*Though, it should be noted, that you can ONLY summon other players into your world if the boss of the area you are in is still alive, and killing him will stop both the Dark Phantoms and other players from ever entering your world while you are in that area.
My biggest advice though? Don't sweat the details too much at the start . While in a game that is as delightfully difficult as Dark Souls you do need to be mentally able to handle the inevitable deaths, the game is best when you sort of dive in, find a pattern/style you're happy with, and letting yourself evolve naturally as you being to become giddy as a child at the prospect of becoming that little bit stronger. While listening to others and reading the wiki's are advised as like others say there some things in game that aren't explained brilliantly, don't let those things ruin the game for you.