Someone who's motherly is loving and nurturing. Your favorite teacher all through elementary school might be your sweet, motherly third grade teacher.
When you act the way a really devoted mother would, you can call yourself motherly. Your older sister might be motherly, protecting and caring for you ever since you were little, and your motherly dog might take in a litter of abandoned kittens, grooming them as if they were her own babies. Motherly comes from mother, which as a verb first meant only "be the mother of," but increasingly came to also mean "nurture" or "take care of."