These are a series of videos and documents which will help you to technically understand KRACK. All you need is about 2 hours.
First, you need to understand the 4-way handshake. Marcus Burton is doing a great job explaining it in this video (6mins):
The following videos will have Hemant Chaskar and Vivek Ramachandran explain all of the KRACK vulnerabilities in technical details. Please watch them in order (79 mins).
Finally, read the research paper from Mathy Vanhoefm explaining his findings in details (25-30mins).
At the end of this 2h KRACK learning session, you should have a better technical understanding of the different vulnerabilities.
Thank you!