Overview
What it does: Helps you find ores.
Why you should use it: It encourages and rewards manual mining. Some popular mods almost seem to be taking the 'mine' out of Minecraft (quarries, etc). I'm trying to put it back in.
How it works in 15 seconds: Craft an Eye of Prospecting, combine the eye with an ore you want to find, and use the "tuned" eyes to search for ores like you use an Eye of Ender to locate a stronghold. If you really like a specific ore, turn 4 Eyes of Prospecting, which have been tuned to your ore of choice, into a Compass of Prospecting that you can turn on and off with right-click, and change the sensitivity of with shift + right-click.
Illustrated Guide
Make an Eye of Prospecting
Tune the Eye of Prospecting to an ore
Find ores with your tuned Eye of Prospecting
Make a Compass of Prospecting
Find ores with your tuned Compass of Prospecting
Repair the Compass of Prospecting
Icon credit: https://www.instagram.com/lowkohart/