A habit is a behavior that is performed regularly, sometimes automatically, often unconciously.
Our brain evolved to "implement" habits to reduce the cognitive load of problems that arise over and over in your environment. You can think of them as quick and reliable solutions to recurring problems.
Habits are formed and maintained via the Habit Feedback Loop.
It's obvious that habits can be positive or negative, but maybe it's not so obvious how much this characterization dependes on the context - smoking is positive in that it calms your nerves, but has negative effect on your health. Exercise might be temporarily unpleasant, but has a positive long term impact. Seeing how Continuous Improvement has a direction you are heading at its core - we can agree to say that positive habits are generally the ones that bring us benefits in the long term.
Our habits determine our outcomes. Even though they seem like a small part of our lives, especially when considering some big goals we have, habits set a trajectory in which you move - moving a meter forward is better than moving 100 meters backwards or not moving at all. See 01 Inbox/Kaizen.