What does the US visa photo mean?

It means 5 centimeters by 5 centimeters.

You can take photos in photo studios, but never take photos in front of the embassy. You won’t get the wrong photo, but it will be very ugly. There may be some good ones, but the ones I’ve seen are all just for you. Standing in front of a white wall and taking pictures with a camera worse than my digital camera, and then printing them out with a printer, I might as well take the pictures at home. But if you don't take care, it won't be a problem.

I used to take photos of my wife at home, choose a good-looking one, print it out on photo paper and cut it out myself. As long as they are about the same size, with a white background, front view, both ears exposed, and the head area accounts for about two-thirds to three-fifths of the photo, there is no problem. You can cut it from your previous photos, but most people don’t have photos that comply with the regulations, so it’s generally not possible.