A 'top to bottom' conference in basketball is when there are many good teams in a single grouping. Rather than having a select few elite teams that dominate the regular season, top to bottom conferences have several talented teams that can win on any given night and ultimately contend for a championship.
Conferences are used to divide all the teams in a league into two separate groups, primarily to make scheduling of games easier. Teams that reside in the same geographic region are usually grouped into the same conference, although exceptions exist in some cases. The NBA's 30 teams are divided into two conferences (Eastern and Western), with 15 in each one.
The success of a basketball team is almost entirely based on the team's win-loss record. Teams with a large number of wins and few losses are generally regarded as 'elite,' while teams that fail to compile more wins than losses are considered below-average teams that usually find themselves at the bottom of the standings. Some of the NBA's best teams at the conclusion of 2018-2019 regular season were the Golden State Warriors (57-25), Milwaukee Bucks (60-22) and Toronto Raptors (58-24).
For the past few seasons, the NBA's Western Conference has been a much better representation of a top to bottom conference than its Eastern counterpart. Whereas the Eastern conference boasted only two truly elite teams and the rest posted noticeably worse win-loss records, the Western conference had several teams with over 50 wins at the conclusion of the regular season. The very best teams in each conference were comparable, however there was far more consistency in the Western Conference. The top eight teams all had strong records and posed a threat to their in-conference rivals on any given night. For example, the difference in roster talent between the first place Golden State Warriors (57 wins) and fifth place Utah Jazz (50 wins) was minimal compared to the gap between the Eastern Conference's top-seeded Milwaukee Bucks (60 wins) and fifth-seeded Indiana Pacers (48 wins).
Top to bottom conferences tend to help the league as a whole, as they prevent an upper echelon of teams from dominating the rest of the conference. Viewership ratings are higher as a result, since neutral fans prefer tightly contested battles to blow-out victories.