Depth of Field Preview Button Explained! (aka that mysterious button below your DSLR)

What is that little button on the bottom of your DSLR that does seemingly nothing?

Gear I use to film my YouTube videos:
https://kit.com/zyproductions/youtube-filming-kit
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Видео Depth of Field Preview Button Explained! (aka that mysterious button below your DSLR) канала ZY Productions