Depth of Field (DOF) is a description of how much of your picture / video is in focus.
A shallow depth of field simply means that one specific area of your image is tack sharp while other elements remain blurred.
Factors for getting Shallow depth of field
Lenses with lower F (Canon 85mm F1.2 would give better shallow depth of field compared to 85mm F1.8)
Cameras with larger sensors (Canon 5d mark 3 (full frame) give better shallow depth of field compared to Canon 7d (APS-C sensor)