I've found some questions and those two badges "Epic" and "Legendary" regarding the "daily reputation cap". What is it and how can I hit it?
The maximum amount of reputation you can earn in one day from upvotes and approved suggested edits is 200. Any upvotes you receive after reaching that number no longer award any points.
You can still earn rep beyond the cap by winning a bounty, accepting an answer, or having one of your answers accepted. Reputation earned from these activities before reaching the cap does not count towards the cap. Downvotes still count against you after reaching the cap, but when this causes your score to drop below the daily cap amount, the points lost can be reclaimed from new upvotes. (Previous votes will not fill in the difference.)
If any votes to your posts are invalidated by the system (e.g. by the serial voting script), your reputation will be recalculated as if the invalidated votes never happened. For example, if a serial upvoter caused you to hit the cap, and you later earned legitimate upvotes that didn't give you any reputation, you will earn the reputation from those legitimate votes once the script reverses the serial votes.
The reputation cap is per site, so reaching the reputation cap on one site doesn't influence reputation that can be earned on other sites.
As time goes by, the above answer is no longer 100% correct. Specifically,
Reputation earned from these activities (accepting/accepted/bounty) before reaching the cap does still count towards the cap.
Has been changed. What's in effect now is
Reputation earned from these activities before reaching the cap no longer counts towards the cap.