Run groups are a way to organize runs on a step into named sections. A new run group can be added to a step in plan or execute by clicking on the run group drop down in the bottom panel. By default all runs are added to the 'default' run group on a step, thus the run group drop down is labeled 'default':
Run groups are specific to each step and all interactions made in the bottom panel (adding runs, uploading files, etc) are specific to the current run group. Each run group can have up to 8,000 runs.