Depth of Field Preview
A control that stops the lens down to the working aperture so the actual depth of field can be seen before exposure.
In depth
Lenses normally focus and compose wide open for a bright finder, closing to the set aperture only as the shutter fires — so the finder always shows the shallowest possible depth of field. The preview button closes the iris early, at the cost of a much darker image. On mirrorless cameras the gain in brightness is handled electronically, making the preview far more usable.
Examples in practice
Pressing preview reveals that a distracting fence, invisible at f/1.4, becomes clearly recognisable at the chosen f/8.