腐 is a bound form (b.f.) that never stands alone in modern Chinese—it always appears as part of a compound, where it contributes the core sense of decay or corruption, whether physical (rot, putrefaction, staleness) or moral (corrupt, degenerate). You'll encounter it most often in words like 腐败 (corruption, to rot), 腐烂 (to rot, to decompose), and 腐化 (to corrode, to become corrupt).