Flip-up backup iron sights, often shortened to BUIS, are a purely mechanical fallback for a rifle that normally wears an optic. A front and a rear unit clamp onto a Picatinny rail. Most of the time they stay folded flat, sitting low so they do not clutter the view through the optic.
If the optic stops working - a flat battery, a cracked lens, a knocked-off mount - you raise each sight until it locks upright and use the front post and rear aperture instead. Nothing needs power, and the swap takes a moment once you have practiced it.
Set-up is straightforward: tighten each unit on the rail, zero the irons once at the distance you use most, record the windage and elevation settings, and re-check zero any time the sights are removed. An example product page for these flip up sights lists the details of the Ozark Armament set.
Backup irons are most often paired with a red dot. With both sights raised, look through the dot's window and see where the irons fall: centred on the dot is an absolute co-witness, while irons in the bottom part of the window is a lower-third co-witness. Confirm which one you have before relying on it. One option to look at is this red dot sight; check its mount height against your irons.
Always follow local laws and your range's safety rules.