Okay kiddo, have you ever seen a movie or a TV show on a big screen? Well, before it gets there it goes through a process called encoding which basically means turning the pictures and sounds into a digital code that can be sent over cables or through the air to your TV.
Now, SMPTE 344M is a fancy name for a standard that describes how this encoding process should work for a certain type of video called high-definition video. This standard tells us exactly how to encode the video so that it looks and sounds amazing on your big screen TV.
Think of it like a recipe for cooking a yummy cake. The recipe tells you what ingredients to use, how much of each to use, and how to mix them together to make a delicious cake. Similarly, SMPTE 344M tells encoding machines what data to use, how to compress it, and how to send it through cables or airwaves so that your TV can display amazing high-quality video images.
Hope that helps you understand what SMPTE 344M is, kiddo!