Your humors are the stats that may be spent for different actions such as building, combat, laerning etc.. They are represented with the lines of different colour at the top of the screen.
Phlegm (blue) is used for energy-consuming actions like running, swimming, sawing boards etc. You may think of it as your physical strength and endurance: the more you have it, the longer you can run, work etc.
Blood (red) is your health. If you recieve damage - you will loose blood. Once it is 0 - you will get KO'd. If you engage in combat with an animal - it will not kill you (you remember that in this game you can permanently loose your character if it will be killed). The animal will KO you and you will spawn at your base losing your inventory (but not the equipped items). However, players and some late monsters from the darkness can permakill you - so beware and trust noone! But dont fear - you can not face monsters that are able to permakill you in the beginning of the game. But you must know - every time you KO you lose half of your current inspiration and 2 random proficiencies are lowered by 1. (I'll tell about them futher). Also, you you were bitten by an animal you might start bleeding - and therefore loosing your blood, which also can result in a KO.
Yellow bile is your combat stat. The more you have it - the greater damage you deal with your attacks. Also, you can turn on movement type "forage" wich will give you the maximum possible chance of finding foragebles and will consume your yellow bile. The less of it you have left - the less damage you deal, so don't engage in a row of hard combats without a rest. Deers (and rarely bears) can break your bones, lowering your YB greatly - to heal you will need a splint (bones won't heal by themselves).
Black bile is your mental abilities, sanity - as it is used when you study inspirationals or place building projects (signs). The more black bile you have - the more you can learn without a need to eat, the faster you gain inspiration and the higher is maximum cap of how much of inspiration you can have. Hoover your mouse oner inspiration number in game to see the cap and how fast do you gain inspiration. Also, if you are scared by a bear you may be KO'd by fear, which consumes your black bile.