It is to face and be brave in an unpleasant situation. Example: I hate waking up early in the morning, but I'll just have to bite the bullet.

When a person has to do something despite it being something that they do not want to do, we say "Bite the bullet". It means that one should endure a bad situation when it is unavoidable. It has a similar meaning to "Just deal with it".

It's an unpleasant reference to the American Civil War. There were no anesthetics--surgeons were judged on their ability to perform excruciating procedures as quickly as possible--and patients were given lead bullets to bite on--the lead being soft enough to yield slightly to the teeth--thus allowing hard biting, which helped cope with the pain, without damaging the teeth. Well, that's what I used to think, but Wikipedia says the phrase is first recorded by Rudyard Kipling in 1891 and that there's no good evidence that patients bit bullets rather than leather straps. In any case, figuratively, it means to accept something extremely painful.