What does this stat mean

Depends on your view point.   When the draft was many more rounds all these guys would have been drafted and considered great finds in later rounds.  So it should still be a positive IMO.

That does seem like an unusually high number. I wish they wouldn't throw numbers out there without giving it some context. I wonder where it stacks among the rest of the NFL. OK, so I googled it. Here's what I found: 

Shefter tweet: "There are more undrafted free agents on NFL rosters today (481) than 1st- and 2nd-round picks combined (480), per Elias." This was last September, but I would imagine the number stays pretty close to that each year. So 481 / 32 = 15.03. 19 is higher than average, but probably not crazy high. 

Thanks MB. I would wager a better way to look at it revolves around the "contribution" factor.  Measuring starters, primary subs rather than just being on the roster playing special teams.

This stat really tells us that talent evaluation is suck a guessing game.  The raw numbers cannot tell you what a person's drive and heart are going to be like in real game situations.

It would be interesting to see if the league-wide number has changed over time, at least since the draft was cut to 7 rounds. 

Someone could pose a hypothesis that in a culture giving so much instant gratification to young people, the players who get a fairly big contract (first few rounds) are more likely to slack off after getting drafted.