HTML What does the alt attribute do?

I'm kinda confused how the alt attribute works. It can be used for images and videos and w3schools (the site I use to learn HTML) isn't properly explaining it. First, the site used an image and hade the alt attribute decribe the image like this: alt="Girl wearing a jacket" But later it does alt=" https://www.w3schools.com/ " I'm so confused. Alt means 'alternative text', but how does that work? I'm stru