The Thu'um, also referred to as Storm Voice or simply the Voice, is a form of magic that most Nords and some others possess which utilizes the Dragon Language to form Dragon Shouts of immense power. The word itself directly translates to the word "shout" in the Dragon language. Most shouts are made up of three words of power.
Well-known bearers of the Thu'um include, but are not limited to, Tiber Septim, otherwise known as Talos Stormcrown, Miraak, Ysmir Wulfhearth, Ulfric Stormcloak, The Tongues of the Dragon War, the Greybeards atop The Throat of the World at High Hrothgar, and Svarog, the most recent Dragonborn in history.
Anyone can learn the Thu'um with enough time and dedication. However, a person marked as Dragonborn has an innate skill with the Thu'um making it easier to learn and with less time dedication.
A person who learns the Thu'um can use Shouts that they have learned. After using a single shout, they must complete a long rest before they may use another Shout.
A novice Dragonborn may use any of his/her shouts twice before needing to complete a long rest. As a Dragonborn gains more control and power, they will be able to use Shouts more often as well as gain other benefits. A Dragonborn's power advances as follows:
Upon receiving the knowledge of their first shout, engaging in their first combat with a dragon, or otherwise having their first introduction to the training of the Thu'um, the prospective Dragonborn rolls 1d100. There is a 5% chance (96-100) that they gain the power of the Thu'um. At which point, they become a Novice. As a Novice, they may use 2 Shouts per day.
Novice: 2 Shouts per day
Acquired after consuming 10 Dragon Souls; Acolyte: 2 Shouts per day, Can speak or yell their shouts, for diversified effect.
Apprentice: 3 Shouts per day
Adept: 4 Shouts per day
Master: 5 Shouts per day
Grandmaster: 6 Shouts per day, Can Whisper, Speak, or Yell their shouts, for diversified effect.
(The Thu'um is not an ability, but a parent skill of Shouts. There is no effect to create from the Thu'um, just each individual Shout's abilities. Like a spell, many Shouts have a saving throw to make, and the DC for each shout is determined as such:
8 + your proficiency bonus + your Charisma Modifier.
If an attack roll is required by a shout, likewise, your attack modifier is your proficiency modifier + your Charisma modifier. )