Place something underwater, and the pressure of the water all around it will try to push the water in to any small gaps or spaces in the casing.

Depth is important because the deeper underwater you are, the greater the pressure you experience will be - so the deeper you go the harder the water is trying to be pushed into the device.

Time matters because once you reach a pressure where fluid can leak in, this still takes a while to happen. If your gasket is 1mm thick, and the water will be able to push through that seal at a speed of 0.1mm/minute, then your device will stay dry for up to ten minutes, but after that point the water will potentially have compromised it.

So when manufacturers test a device, they choose a pressure and time to test for and to rate the device as.