A bubble over in basketball is when a team is on the cusp of making or missing a basketball tournament or playoffs. Several teams will be referred to as "on the bubble" nearing the end of the regular season.
In the NCAA, bubble teams are those which have a shot at making the March Madness tournament, but may not qualify. For these teams, it's important to perform well in their conference playoffs to jump up in the rankings or even win the championship for an automatic bid.
If a team does qualify, they will likely be a 6, 7, or 8 seed. These are the three lowest seeds, so they will have tough opening round matchups. Many bubble teams will miss the playoffs, ending up as a 9, 10, or 11 seed.
In the NCAA, many teams earn automatic bids by winning their conference championship. This can affect whether or not bubble teams make the tournament. If a team who wasn't going to make the tournament earns an automatic bid, it could push a bubble team out of the tournament. Specifically, it would hurt the bubble teams in the same conference who are missing out on the automatic bid.