Save it up for when you need help with a boss, use it, become human, and summon another player for assistance.
Using Humanity:
Regaining Human form costs one Humanity point, which you must spend at a Bonfire.
You may choose to "Kindle" any bonfire by spending a Humanity point there, while you are already Human. Doing this will increase the maximum number of "swigs" your Estus Flask recovers from staying at that fire.
Human form will allow players to summon Phantoms for help and guidance on their journey.
Human form will allow players to invade another player's world.
Human form will also leave the player open to invasions by hostile Black Phantoms.
Stacking Humanity:
The amount of Humanity held by the player is represented by a number in the top left corner of the HUD. Usually referred to as 'soft' humanity.
"Holding" Humanity - that is, when you have Humanity on your person (in your Humanity meter) - puts those points at risk. Upon death, your held Humanity will be left with your bloodstain in addition to your Souls, thus increasing the price of dying twice in a row.
Holding Humanity is not the same as being human - recovering your Human form will cost you one Humanity point from your meter.
Humanity affects the damage of weapons and spells that do "Chaos" damage. Maximum at 10 Humanity.
Humanity increases the player's Physical and Elemental Defenses (not Bleed and Poison Resistance), similar to leveling up.
Humanity increases the player's resistance to being Cursed. Maximum at 30 Humanity.
Humanity also increases the player's Item Discovery rate, which determines the likelihood of enemies dropping items. Maximum at 10 Humanity.
At 10 humanity the item discovery rate (no other enchantments or buffs) is 210. (patch 1.05 and non-patched game)
Maximum Item Discovery rate is 410 and is achievable by holding 10 humanity and using the Covetous Gold Serpent Ring or the Symbol of Avarice .