This video is a closer look at the Basic Armbar from Closed Guard. Although the title has the word "basic" in it you will see that there are a lot of steps involved. Being honest there are a lot of other armbar variations that are easier than this armbar but teaching this at an early stage focuses on a lot of fundamental BJJ movements like staying tight, elevation of the hips, rotation of the body, domination of the arm, squeezing the knees and not crossing your feet.
I find that although there are a lot of steps it's not so far advanced that first day students cant get it. It does require practice and drilling (just like every other move) but as I said before it is taught early in the curriculum so that we can get some transfer learning out of those strong fundamentals.