Your goal in Mario Kart Tour is to beat your rivals and come in first place, but that's never easy when you're also trying to avoid banana peels, red shells, flying bombs, and other items. One item you may be familiar with is Blooper, a squid-like creature found in most water levels. It squirts ink all across your screen, making it more challenging to navigate your driver on the track.
The Blooper item is a recovery item, which means that when a player launches it, it hits everyone in front of them. No one in front of that driver can avoid the ink all over their windshield. While the ink will wear off after a few seconds, that can be the difference between first and last place.
While you can remove it prematurely by using a Dash Panel or a Mushroom, going underwater, or leaving the water, none of those options are always readily available — which is why you can use your finger to wipe the ink off the screen instantly.
It may not be obvious, but all you need to do is swipe your finger anywhere there's ink on the screen, and it will instantly wipe off. You do need to be careful though since your finger is also used to control your driver. But if you're careful enough, you can both wipe the ink and get some boost from drifting on the track. Below you can see the trick in action on an iPhone (it also works on Android).
Beats looking at the corners of your screen to not crash into anything.