Understanding of concurrent IO

see a saying on the Internet

< hr >

Sequential IO mode (Queue Mode) / concurrent IO mode (Burst Mode): disk controller may issue a series of IO commands to the disk group at a time, which is called sequential IO; if the disk group can only execute one IO command at a time. When the disk group can execute multiple IO commands at the same time, it is called concurrent IO. Concurrent IO can only occur on a disk group consisting of multiple disks, and a single disk can only process one IO command at a time.

said in another article:

< hr > If the concurrency of

IO is low, the% iowait is high; if the concurrency of IO is high, the% iowait may be relatively low.

what I want to ask is:
what does the disk group here mean? Usually we use the web server, is the disk used on a single machine a single disk or a disk group? Does it support concurrent IO?
Why "if the concurrency of IO is low,% iowait is high; if the concurrency of IO is high,% iowait may be low." ?