First of all, Stash RPG does not provide a routine "restore over time" feature. Many new players get surprised on why waiting so long and having no Mana restored. This is the fact: Mana will never normally regenerate over time. Regenerating Mana over time is only possible when inside the Rolling Stone Inn, or when using different kinds of camping bivouacs, either way this process uses up considerable amounts of time. Also, when using camps you are risking being ambushed by a group monsters. Therefore its is much more convenient to do "one click restore": instead, foods has to be eaten or potions drank, which makes procuring them the most important task in leveling a character. In combat, you cannot use food but only potions, they take up one turn to use and you are only allowed to drink one potion per so many turns. Potions are not given separate cooldowns, so bringing several tiers of the same potion won't help at all, instead, bring the best one you got. Potions have huge joint cooldown in combat but not outside, where they can be spammed. Playing the Healer class greatly enhances mana restoration (see Funeral Pyre), although Healers have certain hold-ups when soloing due to weak aggressive DPS.
Mana is increased permanently when you level up, or increases temporarily when you are buffed by an ability buff or a potion, or wear a gear piece with enchantments adding to Willpower and Intellect, to an extent of 5 per character level for both ability/potion buffs and gear enchantments. It is consumed when you use abilities. It is possible to play without using any abilities in combat by only using the Basic Attack 
(melee weapons or fists, you will swing a bow like a club using the Basic Attack). Some high level abilities consume great amounts of Mana. Therefore combat tactics should be executed and foods or potions should be used to quickly restore Mana after almost every fight.
As stated above, Willpower and Intellect both add to Mana.