What is Bug
A flaw that makes software behave incorrectly. Finding and fixing bugs is a permanent part of every developer's job.
In plain terms
Bugs appear in all software at every quality level. A regression is a specific kind: something that used to work and broke because of a later change. Debugging skill, meaning diagnosing an unfamiliar failure quickly, is one of the strongest practical differences between developers.
What it tells you about a developer
Great debuggers reason systematically when the cause is unknown, and their war stories are specific: the symptom, the hypothesis, the diagnostic step, the cause. Small, precise fixes accepted into critical code are the visible trace of that skill.