Well, here's what the refpage says. It's a clearer explanation than I could manage on one cup of coffee....
When a new value is received, object filters an input value logarithmically between changes using the formula
y (n) = y (n-1) + ((x (n) - y (n-1))/slide)
A given sample output from slide is equal to the last value plus the difference between the last value and the input divided by the slide value. Given a slide value of 1, the output will therefore always equal the input. Given a slide value of 10, the output will only change 1/10th as quickly as the input. This can be particularly useful for lowpass filtering or envelope following.