Lexicographic means dictionary order, character by character. Compare first characters: if different, smaller one comes first. If equal, compare second. Continue until difference or string ends.
If one string is a prefix of another, shorter comes first. So "app" < "apple" because "app" ends while "apple" continues. Shorter is less than its longer extension. Character comparison uses ASCII values.
Uppercase (65-90) comes before lowercase (97-122). So "Zebra" < "apple" because 'Z' (90) < 'a' (97). This can surprise if you expect case-insensitive order.