Meditation is not the means to an end,
it's both the means and the end.

- Krishnamurty

Meditation allows you to replenish your mana pool after you've casted spells. It is an essential skill for anyone who intends to make an extensive use magery either through combat, marking or scribing. You can meditate actively or passively. When actively meditating (ie using the skill), any action you perform in the game, other than moving items, will break your concentration: you cannot dress/undress, arm/disarm, open containers, speak (other than through party) or move. If you get hit either physically or with a spell, you will get knocked out of meditation. As most skills, there is a delay before you can use meditation again.

The type of armor you wear will affect your ability to meditate. You cannot meditate if you wear any piece of metal or studded armor unless they specifically say mage armor. Along with Meditation, Focus is the other skill that will help you regain mana. While Meditation allows you to regenerate mana faster, it has 2 downsides compared to Focus: A) the highest regeneration will occur while actively meditating, which means you must stop everything you're doing to meditate. If you get hit while meditating, you will lose your concentration and will have to wait before resuming meditation; B) You may occasionally fail to meditate, which means you will only benefit from the extremely slow passive mana regeneration until the delay before your next use of the skill is up. The lower your mana is before you attempt meditating, the higher your chance of failing active meditation since your success rate is based on the difference between your highest potential mana and the level you're currently at.

Compared to Focus, Meditation also has another downside: it only regenerates mana, whereas Focus also regenerates stamina. However, Focus, though always passive, regenerates mana much slower than Meditation, so if you heavily rely on mana, Meditation is the skill to go with. But if you have the room, a combination of Meditation and Focus would be ideal since the mana regeneration of both skills stacks. Wearing mana renegeration armor/items is also a big plus.

Training

This is a breeze to GM. Jump on a boat and sail slow forward heading south or north and cast any spell on the tillerman or yourself.  Use up all your mana.  You will gain faster when your mana pool is at its lowest.  You could use the 8x8 method but since meditation is continuous, you will automatically gain as your boat sails over the next gain tile, 8 tiles further. You do not need to be actively meditating to gain.

Quick tip: getting drunk will help raise your mana passively at lower levels. Also, adding Lower Mana Cost (reduces the amount of mana required to cast a spell or do a special move) and Mana Regeneration (accelerates the speed at which your mana replenishes itself) pieces to your armor is a huge advantage for any spellcaster or mana user.