kccrow's answer was very good.
If you want some additional info, try these.
Written in 2013 but still pretty good -
https://overthecap.com/a-guide-to-the-nfl-salary-cap/
Some of the rule changes for 2020 -
https://overthecap.com/explaining-so...ules-for-2020/
Done in a question and answer format, but good information -
http://www.askthecommish.com/SalaryCap/Faq.aspx
Of course the ultimate source, but lots of reading, is the NFL CBA, Article 12 and 13. It begins at page 61 and goes to page 107.
https://nfllabor.files.wordpress.com...-2011-2020.pdf