Mana is a statistic which is necessary to cast a champion's abilities. Your mana is shown as a blue bar underneath the health bar (green bar) and has two numbers representing your current mana our of your total mana and your total mana. Your mana will regenerate over time up to the maximum amount of mana you can store.
Related Threadswhat is the fastest way to get more mana? - last post by @ Oct 11, 2004
Mana Formula? - last post by @ Aug 16, 2009
mana - last post by @ Jan 31, 2008