A mirrorless camera works, well, without a mirror! Unlike the DSLR, it does not have a reflex mirror reflecting light into an optical viewfinder or passes it directly through to the camera sensor. Instead, light passes straight into the lens directly onto the digital sensor, which shows you a preview of the image on the camera’s LCD screen.
This allows you to see the picture and how it will turn out and make adjustments accordingly. This is why they are compact as well, as space is not needed to fit a mirror. Mirrorless cameras have been made simpler, lighter, and less bulky compared to the cameras before them, and this simplicity is why they are taking the photography world by storm.