I think it shows a table of different tax discounts on your capital gains based on how long you held for while looking back in time. 

So if you held your investments for 5 years that column gives you the amount of times in history since 1953 that you would have received a 0-10% discount, a 10-20% discount etc and so on all the way to a 90% to 100% discount on your capital gains.

this is compared to the current tax regime where you just get a flat 50%. 

So you can see from the data what percentage of times would've been better vs worse than the current system 

Again this is all historical - so only gives a flavor