Stats Window
The stat window lists your primary and secondary stats, and it lets you improve your stats by using the stat points you receive every time you level up.
In order to open your stats window, click on "Status" on the top bar, if it's not already pressed, to open your Status Window. Then, in your Status Window (which is at the bottom left by default), click on "Stats".
You can hover over the values of the primary and secondary stats to see a detailed description appear. This is particularly useful for stats like Attack to see your attack range and base damage per second, Magic in order to see your magic range, or Defence to see how much damage is reduced at your level from certain physical/magical attacks.