A 'top heavy conference' in basketball is a group of teams in which there a few elite teams at the top of the standings and the rest are below-average. This disparity is usually caused by an immense amount of talent on teams with a higher budget to spend on player contracts, and is most noticeable in the NBA.
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 Eastern Conference has been far more top-heavy than the Western Conference. Whereas the Western Conference was home to several teams with more than 50 wins, the Eastern conference boasted only two truly elite teams, with the rest posting noticeably worse win-loss records. 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. The same could not be said for the East, where teams in the middle of the standings weren't nearly as talented as the conference's top teams and generally struggled to be competitive in head-to-head matchups.
Top-heavy conferences tend to hurt the league as a whole, as they present far too many situations in which the elite teams defeat the less skilled teams with ease. Viewership ratings suffer as a result, since neutral fans would much rather see a tightly contested battle than a blow-out victory on a consistent basis.