What title is given to Macbeth for his bravery in the battle?

In act one, scene two, the Captain elaborates on Macbeth's bravery during battle and describes to King Duncan how he defeated Macdonwald and the Norwegian king's forces. When Ross enters the scene, he tells King Duncan that the Thane of Cawdor is a disloyal traitor, who assisted the Norwegian king in battle against Macbeth and Banquo. After learning this information, King Duncan announces that Macbeth will be given the new title, Thane of Cawdor, and the former Thane of Cawdor will be immediately executed. In the next scene, Macbeth and Banquo meet the Three Witches on the heath following the battle and listen to their presumably favorable prophecies. The Three Witches address Macbeth as the Thane of Cawdor and future king. Shortly after the witches disappear, Ross and Angus arrive on the scene to inform Macbeth that King Duncan has given him the new title, Thane of Cawdor, which trills Macbeth and incites his ambition to become the king of Scotland.

To answer this question, take a look at Act I, Scene II of Macbeth. In this scene, King Duncan listens to an eyewitness account of the battle against the rebels. In particular, he listens to the report of Macbeth's bravery on the field as he overcame the rebels and killed Macdonwald.
As a result of his bravery, King Duncan rewards Macbeth by making him the thane of Cawdor. This title is important because it not only demonstrates Duncan's gratitude but it also adds significantly to Macbeth's prestige since he is already the thane of Glamis.
Macbeth, however, is not present in Act I, Scene II when Duncan bestows this title upon him. So when he finally hears the news in the next scene, he believes that it validates the witches' prophecies. This sets in motion the major events of the play, including the murder of Duncan.