It says "Hit the daily reputation cap on 50 days."
What exactly does it mean?
It means you have hit the daily reputation limit of 200 rep points 50 times. This can be accomplished consecutively or non-consecutively.
You can only earn up to 200 reputation points per UTC day. Accepted answers and bounties are not limited to the cap.
To clarify the definition, it means you ended the day with 200 reputation or higher.
It's possible to hit the cap during the day and then lose rep later. If this happens you don't get credit towards epic/legendary for that day, even if you "left rep on the table" from upvotes received while you were at the cap.
This hits me particularly hard, as it's the difference for me between already having the legendary badge and still being some 40 days away.