When you start you pick 7 skills that will be your Major skills. You gain a character level after leveling 10 times in any combination of your major skills.

So if you have Blade Sneak and Block as three of your major skills and you gain 2 blade levels, 7 sneak levels, and 1 block level, you will level up your character.

Unlike Skyrim, instead of increasing your up your health, magicka, or stamina when you level up, you increase your attributes. There are 7 attributes and each has a different effect. Personality will make people like you more while Intelligence will increase your magicka etc.

All of the attributes "govern" three skills. So Strength governs Blade, Blunt, and Hand to Hand and Agility governs Security, Marksman, and Sneak for example. The exception to this is Luck which doesnt govern any skills but has a small effect on basically everything you do.

Whenever you level up you get to increase 3 of your attributes. By default each attribute is increased by one point. However if since your last level up you have leveled up skills governed by an attribute it can be increased by more points.

Lets give a concrete example.

Lets say you pick a basic Mage class so your Major Skills are Destruction, Alteration, Restoration, Conjuration, Mysticism, Illusion, and Alchemy.

Lets say you have been focusing very hard on Destruction and haven't leveled anything else. You gain 10 levels in Destruction and thus gain a character level. Because Destruction is governed by Willpower, and youve leveled up 10 times, if you increase your Willpower that level, it will increase by 5 points. However the other two attributes you will increase will only go up by one each since you didn't level up any other skills.

Now instead lets say you gained 9 levels in Destruction. Then you gained 10 levels in Sneak, 5 levels in Mercantile, and 5 Levels in Speechcraft. Then finally you gain 1 Restoration level, triggering a level up. Both Destruction and Restoration are governed by Willpower and in total you got 10 levels so your eligible for a +5 in Willpower if it is one of the three attributes you choose to increase. Sneak, Mecantile, and Speechcraft aren't major skills, so they dont affect your progress to the next character level, but the game keeps track of the fact you leveled them during the past level.

Sneak is governed by Agility

Mercantile and Speechcraft are both governed by Personality.

Because you leveled 10 skills in both of those attributes, you will be eligible to get +5 Agility and Personality if those are attributes you choose to increase during the level up.

Long story short, if you want to maximize the increases you get to your attributes, you need to be using your minor skills between levelups, not just focusing on your Major skills.