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. There are many offerings from sony, canon, nikon, fuji and many more as well as lens adapers meaning you could use your old lenses
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).
Mirrorless cameras allow you to swap and change lenses like a DSLR, but because the mirror inside the camera has been removed, it has allowed designers to make mirrorless cameras much more compact than DSLRs.