Biodegradable means a material can be decomposed by living organisms (bacteria, fungi) into water, carbon dioxide, biomass and other natural substances. The term is widely abused: nearly anything will biodegrade eventually under the right conditions.
For the term to be meaningful, look for a third-party standard with a defined timeframe and environment: ASTM D6400 / EN 13432 (industrial compostable), ASTM D6868 (paper coatings), or OK Compost HOME (home compostable). "Biodegradable" with no standard cited is marketing.
Note: compostable plastics often do NOT biodegrade in home compost or marine environments. Check the actual certification, not the leaf icon.