Electronic tuners are a great tool for keeping your guitar in perfect tune, both with itself, and with the rest of the musical world. There are several different types of electronic tuners on the market, but they all share one common mission: to indicate to you that the note you are playing is perfectly in tune, or too high (sharp), or too low (flat).
You will adjust the tuning peg that corresponds with the string you are tuning to bring the note into tune. Loosening the string will cause the note to get lower, and tightening the string will cause the note to get higher. The tuner will let you know when you've got it spot-on.
The clip-on tuner I am using in this demonstration is the brand name: Snark. It is one of a variety of tuners in my personal collection, all of which I like. I chose this one for the demo because it seemed especially easy for you to see in the video.