What do the tennis-ball ‘countdowns’ on scoreboards mean?

Question

I’ve noticed during the first few Wimbledon matches this year (it may have been there before, but this is the first time I’ve noticed it) that on the court-adjacent scoreboards, right underneath the names of the players, there is a sort of ‘countdown’.

At the start of the game, it seems there are three tennis balls underneath both players’ names. Sort of like this (taken from the Federer/Mannarino game I’m watching right now):

Roger FEDERER
⚫ ⚫ ⚫

Adrian Mannarino
⚫ ⚫ ⚫

At some point during the game – I haven’t been able to deduce exactly when – a tennis ball gets removed from one or the other of the players, sort of like when you lose a life in a computer game. Occasionally, they also seem to get restored.

For example, towards the end of the fourth set in this game, when Mannarino was up 2 sets to 1, the scoreboard was down to this:

Roger FEDERER
⚫ ⚫ ⚫

Adrian MANNARINO
⚫

So two balls off for Mannarino, none for Federer. I’d noticed that it was down to two for Mannarino previously as well.

When the fourth set ended and they were about to go back on court for the final set, however, it was back to being three each – so perhaps whatever it is is reset for each new set.

What exactly does this ‘tennis-ball countdown’ mean?

Answer 1

You're seeing how many challenges they have left (well, how many incorrect challenges they have left) in the set. See for example this RadioTimes article:

Players are given unlimited opportunities to challenge a call in each set at Wimbledon.

However, once three incorrect challenges are made, that player can’t challenge again until the next set.

If the set goes to a tiebreak, an extra challenge is awarded to each player.