The adjective biocompatible, like all words prefixed with bio- (life), means "compatible with living organisms". Biocompatibility is an intrinsic characteristic of a substance (of a material, of a product, etc.) which presupposes that there is no harmful alteration in the vital functions of the living organisms that metabolize it.
We are mistakenly used to thinking that everything natural is automatically biocompatible as well. Nothing could be more wrong. In fact, taking oil as an example, we know that it is composed of a mixture of natural hydrocarbons and yet its effects on the biota (set of organisms living in a given ecosystem), especially in the marine environment, are devastating.
Not to mention what happens to the birds: the oil-smeared feathers will not allow them to swim, fly, defend themselves from predators or be able to get food so they, in an attempt to clean themselves, lick the plumage by ingesting oil and suffering serious damage to the organs. internal that often lead to death.
Being biocompatible means that a substance, whether natural or artificial, when interacting with living organisms, will not harm them or alter their life processes.