I hope that's not one of those apps that perpetuates the "everyone should drink 8 glasses a water of day" myth that sprung up a few years ago (in "research" funded by Danone, who own the Evian and Volvic bottled water brands... quelle surprise).
Most people's diets already contain enough water (2.5-3.5 litres/day) without forcibly drinking extra, so unless you have a specific medical condition, the best advice these days is just "if you feel thirsty, drink some water".
(And yes, "if you're feeling thirsty, you're already dehydrated" is a myth, too.)