To be pedantic for the people still reading this, curious enough to go further without falling down a Wikipedia rabbit hole.

The reflex mirror in a (D)SLR directs the image up through the viewfinder prism to the eyepiece. This mirror flips upwards just before the shutter opens. This mechanic action has a decent bit of inertia and can cause blurring in some extreme cases.

Mirrorless refers to digital cameras where the sensor operates the digital viewfinder, so like an SLR “you get what you see”. For rangefinders and TLRs, your view is offset from the picture lens, so if you’re really trying to nail a composition and not “fix it in post” SLRs and mirrorless offer an advantage, which is part of why they became so predominant.