Intervals are one of those basic things that many beginners are told they need to learn, but are often fearful to do so. I've never understood why that is, personally - intervals really aren't a scary concept in any way. So, I figured I'd do what I can to convey them in a way that's as "non-scary" as possible.
I'm going to stick to a practical explanation as much as I can, focused on WHY you need to learn them and not spending too much time on the sections of theory that are often frustrating to try and understand. This video is also a starting point for a series of lessons I'm doing on modes and improvisation, so if you're not familiar with intervals, you should probably take a look at this so you're comfortable with this concept by the time the next lesson rolls around.