What is a Read Group?

Cited from Read Groups

There is no formal definition of what is a read group, but in practice, this term refers to a set of reads that were generated from a single run of a sequencing instrument.

In the simple case where a single library preparation derived from a single biological sample was run on a single lane of a flowcell, all the reads from that lane run belong to the same read group. When multiplexing is involved, then each subset of reads originating from a separate library run on that lane will constitute a separate read group.