This video is the second part of a tutorial video on GLM and Multilevel in R. It gives a general handwaving introduction, with the main goal of showing the R code.
For a proper introduction into Multilevel modeling as a technique, we recommend this free manuscript Chapter from a great book on the topic:
https://multilevel-analysis.sites.uu.nl/wp-content/uploads/sites/27/2018/02/02Ch2-Basic3449.pdf