League of Legends guide for a beginner: what is a champion?

Champion statistics – defensive and utility stats

In addition to these offensive statistics, there are five types of defensive statistics: armor, magic resistance, heal and shield power, health and health regeneration, and tenacity

Armor: The amount of attack damage that will be blocked before the damage begins hurting your champion.
Magic resistance (MR): The amount of magic damage that will be blocked before the damage begins hurting your champion.
Heal and shield power: If your champion is able to heal or shield himself or a teammate, how much more effective those heals or shields will be.
Health (HP) and health regeneration: The champion’s health pool dictates how much damage they can take (accounting for armor and MR) before they die and are forced to respawn. Health regen is how much health your champion will heal at a given time.
Tenacity: When a champion is crowd controlled (for instance, stunned, slowed, or rooted), tenacity will reduce the effectiveness of that crowd control.

Finally, there are three types of utility statistics: movement speed, cooldown reduction, and mana and mana regeneration (alternatively energy and energy regeneration).

Movement speed (MS): How quickly your champion moves.
Cooldown reduction (CDR): Reduces the time that your abilities are on cooldown after being used. Abilities cooldowns will decrease as they’re leveled up, but getting cooldown reduction can reduce this downtime further. You can only have 45% cooldown reduction max.
Mana and mana regeneration: Champions will need a certain amount of mana to cast their abilities. If they do not have the required amount of mana, they will not be able to use abilities, so it’s important to get as much as you can or have enough regeneration that you will be able to restore the mana expended on abilities quickly. Alternatively, some champions will not use mana but will use resources like energy, which cannot be bought like mana but regenerates far quicker than mana does.

Your champion will get stronger the more of these stats he is able to acquire through items and levels. In addition, some abilities will get stronger as you gain more of one particular stat.

It could be obvious, like a fighter whose abilities improve as you get more AD or a mage whose abilities do more damage as you get AP, but some will scale off other abilities like health, armor, or mana. Which type of stat matters most to your champion depends on what class it is.