I'm working from memory here as I haven't used tt in a few years, but it's basically how far you want to let prices drift from your target spread price before requoting the legs. Fuzzy here- but I remember inside and outside slop. Inside being a better spread price and outside being worse. This helps manage things like fill ratios where you can get penalized for excessive orders and losing your place in the order que when updating your price.
So for example if you set inside slop to a few ticks and outside to 0 you are saying don't requote the legs if the spread price (not really the spread price but the other leg) ticks inside the target price, but requote as soon as the other leg moves outside your target price. So it's really creating a range of acceptable spread prices. Setting both to 1 tick means you're OK with a spread price 1 tick above or below your target.
Hope that helps.