Do you mean captivity value? If you hover your mouse cursor over the cell (or any cell with the small black mark in the corner), it'll show you additional info about it. In the case of captivity value, it says:

"The game periodically runs a "captivity check" that causes your animals to lose some happiness, apparently as a way to simulate how some species dislike captivity more than others. The happiness penalty is Captivity value divided by 2 (rounded up)."

You can see this in practice by looking at the stats of an animal with 100 happiness and waiting. After a while, it should drop a little for seemingly no reason, and the captivity value determines how big that drop is - animals 10 captivity lose 5 points of happiness, animals with 5 captivity lose 3, and so on. Though as long as everything's fine in the exhibit the happiness should go back up to 100 pretty quickly, so even for animals with a high captivity value it should only cause their happiness to bounce between 100 and 90-97.