This "short" endurance test confirms if you SD card is valid and reliable or if it's a counterfeit that has way less storage space than advertised. It fills your card with carefully designed files that the app can then verify to ensure that what's written to your flash card can be read back reliability. It also measures read & write speed at the same time, because, why not?
It's not the most pretty app because the focus has been instead on real-world testing and good algorithms. It is, however, easy enough to use. The only way to know your card is good is to fill it to the advertised capacity. Typically this takes less than an hour but can be longer if the card is particularly slow or enormous. But this app is happy running in the background (unless you use one of those unnecessary app-killers) while you continue to use your phone. Or, just leave it running overnight. Piece of mind is worth the wait and there are no shortcuts to it!
You can also test your phone's internal storage. With USB-OTG you probably can test thumb drives too, depending on how your phone works.