Rerolling is a gaming term to describe starting fresh. Making a new character, or picking a new class. The term comes from Dungeons and Dragons as well as other traditional pen and paper role play games, where you would use dices to roll up statistics such as Strength, Charisma, etc to create a new character. A lot of things about is random in the game, and hence "reroll" is something you would have to do if your character died.
Rerolling is usually associated with Diablo 2, since the game lacks a respec facility.