辛 is a bound morpheme (b.f.) that always appears as part of a compound or fixed expression. Its two core meanings are 'spicy/pungent' (in 辛辣) and 'hardworking/suffering' (in 辛苦, 辛酸). It also serves as the 8th Heavenly Stem in the traditional Chinese cycle and can be used as a surname, but these uses are less common for learners.