By completing Quests (and to a lesser extent defeating adversaries in combat) you will earn Fame. Gain enough Fame and you will gain an additional Fame Level (to a current maximum of 50). Gaining Fame Levels is important as it allows you to develop your Player Character. With each acquired Fame Level your Player Character automatically gains more Health Points and a higher damage output. In addition you will be able to perform one or more of the following:
Add Attribute Points Edit
Whenever you gain a new Fame Level, you gain additional Attribute Points to spend towards one or more of your Attributes. Open Character Statistics (C) and you will notice each of the Attributes having + and – Buttons next to them. You can click these to increase your Attributes or remove added Points. Make these changes final by clicking Apply underneath.
Learn Skills Edit
When reaching certain Fame Levels, you will be allowed to learn new Skills. Open your Skill Book (B) and then click Learn New Skill. This will open up the Skill Library where you can select new Skills by clicking on them. You will be asked to confirm your selection. Once confirmed it cannot be cancelled. If a Skill is available to learn it will be designated as Learnable, otherwise you cannot learn it yet. Note that as you progress through the game Skill from different Sets become available. You can check this by clicking the Set Tabs.
Add Skill Sigil Slots Edit
When reaching certain Fame Levels, you will be allowed to add Sigil Slots to your Skills. Open your Skill Book (B) and click Add Sigil Slot to open the Upgrade Window. Here you can select the Skill you want to add a Sigil Slot to, by clicking on it. You will be asked to confirm your selection. Once confirmed it cannot be undone.
Note that newly added Sigil Slots are empty by default and need to be filled with Skill Sigils from your Inventory to be actually useful. Added Sigils also need to be corresponding to the Skill Type. Each Skill can have a maximum of four Sigil Slots added to it.
Choose a Discipline Edit
When reaching Fame Level 5, you will be allowed to choose a Discipline depending upon your chosen Archetype (Rogue, Spellcaster, Warrior). Open Character Statistics (C) when you have reached Fame Level 5 and click Select Discipline. You can then permanently select the Discipline you want. You cannot spend Attribute Points if you have the option to choose a Discipline. In such cases selecting a Discipline allows you to spend the points afterwards.
Note that to be able to apply any of the manual changes for Character Progression, your Player Character is required to be Out-of-Combat.
Source: Official Site - TCOS Manual