膝 is a bound form (b.f.) meaning 'knee'. It cannot stand alone in modern Chinese; you must combine it with another character to form full words, most commonly 膝盖 (xīgài, 'knee'), 膝关节 (xī guānjié, 'knee joint'), or 膝下 (xīxià, used in letters to refer to one's parents or elders). You will see it in formal, medical, or anatomical contexts.