Star rating (SR) describes a beatmap's difficulty, which is automatically calculated by the osu! client. Star rating plays a large role in how performance points are awarded. This number can be seen on the song selection screen and on a beatmap's info page.
Main page: Difficulty
As of 27 July 2021, the osu! website automatically assigns a beatmap's difficulty rating colour based on their star rating according to the following spectrum:
In some instances, such as AiMod, difficulty is classified into six level ranges based on star rating as follows:
Note that while the intended difficulty levels of most beatmaps match the colours or level ranges they are given, some may deviate significantly due to certain factors. This is why the ranking criteria offers more decisive ground rules to determine a beatmap's actual difficulty level and spread regardless of its star rating.