What are Manas?

You get a Mana from attacking & winning from an opponent who has higher stats than you.

Mana it's a winner-buff.

As already Angelus007 wrote you can get it from race battles player vs. player in which you are a winner; no matter if you are attacker or victim. The opponent must have higher at least one skill then you.

After a race fight the winner gets a buff for 20 minutes which is basically a temporary skill-bonus. This Bonus depends of you and your opponent.

Example:

In this case the biggest difference between the winner and the loser ( skill points which includes also skills which you get from activated items ) in favor of the victim is skill "Charisma", so the winner gets a Charisma buff for 20 minutes. See the table. The bonus of the buff is calculated as follows:

25% of the Skill difference.

25% of 84 = 21

Result: The winner receives a 20 minutes buff of +21 on his Charisma skill.