A mirrorless camera is one that doesn’t require a reflex mirror, a key component of DSLR cameras.
The mirror in a DSLR reflects the light up to the optical viewfinder. In a mirrorless camera, there is no optical viewfinder. Instead, the imaging sensor is exposed to light at all times. This gives you a digital preview of your image either on the rear LCD screen or an electronic viewfinder (EVF).