I opened a PNG image as a layer in Gimp, expecting the background to be transparent. Instead, the whole squared background is visible and covering the image under it. What exactly am I doing wrong and how can I solve it?
You very likely imported a non-transparent PNG. How did you obtain the PNG? There are web image sites that deliver a non-transparent PNG unless you pay (or where the sample is non transparent, with transparency replaced by the checkerboard pattern)....
You can check the PNG in your browser(s) (
File>Open... in Firefox, drag and drop in Firefox and Chrome). These browsers don't display transparency as a checkerboard so if there is a checkerboard it is part of the image.