Brasileirao
In short
The top tier of professional football in Brazil, consisting of twenty clubs competing for the national title. It operates on a system of promotion and relegation with the Série B division.
In plain words
It is the main football league in Brazil. The best teams play each other to see who is the champion of the country. The bottom teams move down to a lower league next year.
A simple example
A club like Flamengo or Palmeiras competes in this league to win the trophy and qualify for international tournaments.
Why it matters
It is one of the strongest domestic leagues in the world. It showcases massive talent and attracts huge crowds and global interest.
Easy to mix up
Do not confuse it with Italy's Serie A, which has a very similar name.