Digitigrade is a term used for animals that walk on their toes rather than the soles of their feet like humans. Birds, dogs, cats, and other animals fall in the category of digitigrades.
What we would think of their knees is actually their ankles. They are jokingly called "backwards knees" or having "backwards knees syndrome" because we don't realize they are digitigrades and it is their ankles we are seeing.
They do have knees, but they are higher up the leg and often covered by fur or feathers or located so much higher that they are within the body and we can't see them. Artists can sometimes struggle with drawing canines, felines, elephants, and birds because they don't realize they are all digitigrades. So, they can't get the legs to look right when drawing them. They simply walk on their toes and not the soles of their feet like we do and that is where the confusion lies.