Ok, so imagine you have a toy car that you want to make sure works really well before you take it to play with it outside. You have to test it first to see if it can drive well, right?
This is kinda like what people do with mobile devices. They make sure the phone or tablet works properly before it's sold or released to the public.
To do this, people create a bunch of different tests that are designed to check different things. For example, they might make sure the touchscreen works by tapping on different parts of the screen to see if it responds correctly. They might also test how long the battery lasts or how fast the device can download apps or websites.
They use special software to run these tests and see if the device is good or needs more work. If the device fails any of the tests, they make changes or fix it until it passes.
The goal is to make sure that when you get a new phone or tablet, it works really well right from the start without any problems.