Quite simple, your mana strength is also the amount of mana you get back each day. e.g. If your mana strength is 10, you get 10 mana back each day (up to 100 in this case). Useful to equip mana boosting artefacts at the end of a turn.

Visit a well for immediate full regen, or spend the night in a town with a mages guild and all points are back in the morning.

If you make use of the Summon Creature spell, you can only summon from the garrisoned creatures in the nearest town, the point here is that you are limited to the number of spell points remaining. 100 spell points gets you 100 creatures. This also uses half your movement for the day so try to do it near a well.

It is possible to regain mana during combat with one or two heroes special abilities, as well as the fact that Inferno Imps steal mana when their turn commences, and the upgraded imps (familiars) transfer this to their hero. Beware of enemies with zounds of imps / familiars.

Also during combat, Necro heroes can learn "Mark of the Necromancer" (ability - use as a spell), which gives 1 mana point for each 75 points of damage inflicted on the affected creature.

Inferno Heroes can learn "Consume Corpse" (ability - use as a spell), which allows the hero to regain 1 mana for each 30 points consumed (target dead stack on the battlefield after casting the special ability).

Finally, Dungeon Heroes can learn Dark Renewal - if a Dark Magic spell is resisted the hero gets back the mana used to cast it. They can also learn Dark Ritual - and get all spell pints back once a day - can only be used at the start of a day and uses up all movement points for the day.

Details on the Hero abilities can be found on the skillwheel, which I think you have.