A subgenus is a rank in between genus and species, although it is not the only one and not every genus has subgenera (plural of subgenus). When a genus is split into multiple subgenera, one of the subgenera has the same name as the genus, so that’s why there is both a genus Chironomus and a subgenus Chironomus (Chironomus). This subgenus is known as the type subgenus.
See also this explanation:
https://forum.inaturalist.org/t/display-genus-along-with-subgenus-section-etc/14107/11