The 'M' position is the name given to the hip healthy position young babies should sit in inside a carrier. Named so because the shape of baby's bum and legs should make resembles an 'M'. The middle point is the baby's bum, with the thighs sitting above and the shins and feet falling by the side.
It's really important that babies sit in this position when using a carrier, otherwise they could be at risk of hip dysplasia.