Ability Scores
There are 6 ability scores.
Ability scores can be improved when a PC Levels Up.
At Level 1, you choose one ability as your Primary Ability.
A PC's ability scores cannot go above 10 by normal means.
When to Use Ability Scores
Ability scores are added to Checks (d20 rolls). They are sometimes added to other rolls, and sometimes Half their value is used to determine other bonuses.
The six ability scores are the primary way PC's are mechanically represented in the game, and increasing these is the primary way they progress.