While in combat, all sorts of reinforcing substances become indispensible. By default, there are only two slots available for potions, but you can unlock the third one by using your Inquisition points. There are three categories of substances: potions with active functioning, such as healing, regeneration and increasing the amount of available mana, etc. Then there are tonics: these have a passive effect on the character and temporarily provide him with boosts to specified resistances, or increase the value of armor. The last category is grenades that you can use over an area during combat. By default, you only have one healing potion, which is free and can be replenished each time that you visit the base and take rest. As for the rest of the potions, they need to be crafted, using the ingredients that you find (you can learn more about it in the chapter devoted to crafting). To change the potions that you are using, you need to use the brewing table and drag the bottle that you want over to the storage. To brew a potion, once you drag it to the storage, you need to click it (you need to have a sufficient amount of ingredients specified on the right side of the screen).
What needs to be highlighted is that there is a limit as to the amount of potions that you can carry at a time (you can increase the number by spending your Inquisition points). A very important fact: all of the party members use the same potions storage. They only use health potions automatically, but it still disappears from the collective storage.
You obtain new potions by completing side quests, performing actions on the tactical map and as you progress in the storyline. You can find schematics hidden all around the game world. Each of the potions can be improved in order to increase its potency.