The term means agreement between lord and warrior. The lord gives shelter, food and gold; in return the warriors provide protection for their lord. In Beowulf, only Beowulf and Wiglaf, as warriors follow the comitatus rule. They both defend their lord, even when their life is in danger.
Boast is to pledge with honor. Throughout the poem, Beowulf boasts before he fights the monsters.
A thane is a warrior-retainer. Who serves his lord in a battle and is loyal to his lord. For this service, the lord or the ruler will provide for his thanes with food and shelter as well as with gifts and wealth.
Fate is a Christian belief that means every action which happens to men, their birth or death, is already predetermined by God. Therefore, men have no control over when, where, and how they will be born or die. "Fate often saves the undoomed man when his courage is good" is a saying of Beowulf. He says this early in the poem when he describes his swimming contest with Breca.