I am not 100% sure, but i believe it's as follows
You're viewing a mon's checks and counters, basically what users on showdown use to check/counter something (in this case i'll assume it's kingambit).
In the instance of zamazenta coming in on kingambit, 25.6% of the time the kingambit got KO'd (died) and 49.4% of the time kingambit switched out. Add those two numbers and you get 75%. What comes after the +- is the margin of error, basically the deviation from the number that comes before it that the real value could have (both above and below)