The article links to https://us-cert.cisa.gov/ncas/current-activity/2021/07/02/ka... , which says it was Kaseya VSA and links to their advisory.

VSA appears to be a proprietary name. They are usually referred to as RMM tools (remote monitoring and management).

They essentially are enterprise level back doors with good intentions.

Think firewall/antivirus/backup software suite run by a remote team.