Guan

Guan is timed tsumego practice on the Chinese Go site 101Weiqi.

Properly called 棋力测试, 'Skill Test' or 'Strength Test', guan provides a set of 10 problems roughly around a selected difficulty from the categories of life and death, tesuji and semeai. Each problem can only be attempted once and has a time limit of 45 seconds for the whole sequence. Success is achieved if at least 8 out of 10 problems are solved. Clearing a level for the first time will unlock the next level.

It's called 'guan' by the English-speaking community because that word appears in the URL. Probably 关, which means "checkpoint" in this case.