Acid1 is a special kind of test that helps to check if a web browser (the thing you use to visit websites on your computer or phone) follows certain rules for displaying web pages correctly. Think of it like a game of matching shapes: Acid1 has a set of shapes that the browser needs to match perfectly in order to pass the test.
Just like how you need to use the right shape to fit through a hole in a puzzle, Acid1 makes sure that the browser is using the right "shapes" (or code) to display a web page. This is important because if the browser is using the wrong shapes, the web page might not look right or work properly.
So when a web browser takes the Acid1 test, it's like it's playing a matching game to make sure it's using the right shapes to display web pages. If it matches all the shapes perfectly, it passes the test and can display web pages correctly.