They describe how they come to their ratings at https://www.allsides.com/media-bias/media-bias-rating-method.... They also describe what it means to be "left" vs "lean left" vs "right" vs "lean right", etc. See https://www.allsides.com/media-bias/left and https://www.allsides.com/media-bias/right (there are also ones for lean l/r, and center).

Now I'm sure these ratings aren't perfect, but I believe they're more accurate than most people's hunches.