The term Biodegradable or Degradable can be misleading as technically everything is degradable eventually but the time scale can be ambiguous. It could take hundreds or even thousands of years to fully degrade.
However, a product labelled with Certified Compostable will degrade within a certain time frame of usually around 3 months, leaving no microplastics behind.
In a pile of organic waste, there are millions of tiny microbes that consume the waste, transforming the organic materials into compost. In order to claim that a product is fully compostable, the product has to meet all the requirements in the European EN 13432 and/or the US Standard ASTM D6400.
Both specifications require that biodegradable/compostable products completely decompose in a composting setting in a specific time frame, leaving no harmful residues behind.