A effort point, or story point, is an abstract measure of effort required to implement a user story.
In simple terms, it is a number that tells the team about the difficulty level of the story.
Difficulty could be related to complexities, risks, and efforts involved.
Gunner uses 0, 1, 2, 4, 8 or 16 as possible effort point values.
Bugs get 0 because fixing a bug should not effect the team's velocity, which is calculating by summing the teams effort points.
For example, if a team has a velocity of 100, the project manager can plan a sprint by assigning a total of 100 effort points to a given sprint.
This helps give estimations both in terms of timelines and budgets.