me too, if anyone can explain it would be nice.
Create an Aggregate Device to combine multiple audio devices - Apple Support
With an Aggregate Device, you can use audio inputs and outputs from different devices at the same time with Logic Pro, GarageBand, MainStage, and most other Core Audio-compliant applications.
support.apple.com
Set aggregate device settings in Audio MIDI Setup on Mac
In Audio MIDI Setup on your Mac, set the sample rate and enable drift correction for aggregate devices.
support.apple.com
the explanation is clear enough.