These are modifiers for the rule. NC is "no case", i.e. ignore upper and lowercases.
L is "last rule", that means, if that rule applies, ignore every rule that comes after it. Think of it like a return command in a script.
There are a lot more, you might also find R=XXX a lot - here is a list of what's possible: https://httpd.apache.org/docs/2.4/rewrite/flags.html