HPs and FPs let you take damage and cast spells (or use special weapon attacks). Both of these limited resources are fixed; they won’t regenerate over time. Visit a Bonfire to restore your resources, or use items to recover in the field.
Bonfires
Every time your character stops at a Bonfire, you get to restore all of your HPs and FPs (free of charge). You can do this as often as you like, so it’s possible to return after every fight if you wish. However, monsters replenish each time you visit a Bonfire, so be mindful of when you choose to rest, since you must fight any enemies nearby that you defeated previously!
Your Estus Flask
Early in the game, you find an Estus Flask that restores your health when you drink from it. You only have so many uses of the flask, but they are restored when you visit a Bonfire. This is usually going to be one of the items that you keep on your quickbar, because you will use it all of the time!
Be mindful that it takes a couple of seconds to drink from your flask. Drink in combat only when you must, and try to get some distance from enemies before you take a drink. If possible, get behind hard cover so you can’t even be hit by ranged attacks. You cannot dodge or move while drinking, so you’re liable to take damage if any foe gets the drop on you.
If you’re badly wounded, it’s possible to take a couple of swigs back to back by pressing the item button again while you’re already drinking. This “double drink” saves on the animation time and heals more damage quicker than if you drank, did something else, and then took another pull off of your flask.
The Ashen Estus Flask
The Ashen Estus Flask works similarly to the regular Estus Flask, but it restores your FPs instead of health. Use it when you need to keep casting spells or using skills, but have run out of juice.
Flask Ratios
Your Estus and Ashen Estus Flask uses are limited. When resting at the Ashen Shrine, talk to the Fire Keeper and set how many flask uses are for health, and how many are for FP restoration. This can be a tough decision for certain character builds. Experiment heavily with these numbers to find a good balance for your playstyle. Determine which resource is tapping out first. If it’s always one of them, try to change the ratio to favor that flask a bit more.
Upgrades
Your flasks improve throughout the game. There are special items that let you drink more often and gain more per drink from your flasks. Be tireless in your search for these items, because they’ll save your life time and time again.