Plastic is used throughout society for personal products and industrial use. Too much of it is ending up in the wilderness or in water supplies. It doesn't break down very quickly in the way that more natural materials decompose and return nutrients to the soil or water. Instead large pieces turn into tinier pieces and eventually microscopic pieces - but all still plastic which is not nutritious for wildlife or marine animals. Bright colored pieces resemble food and are often found in undigested masses within the abdomens of birds and marine life. Smaller microscopic pieces also negatively affect the health of marine larvae, babies of larger species.
Doing our part to keep plastic out of the diet of our fellow inhabitants on the planet is as simple as 1, 2, 3 - Reduce use, Reuse when possible if plastic is purchased, and Recycle when plastic products are no longer usable - or at least make sure it gets thrown into the trash so it will be added to a managed landfill instead of drifting into the wilderness or water supply.
More information about microplastics and the current difficulties for measuring and reducing microplastic pollution in an international river system see: "Dirty Danube: looming pollution threats to the world's most international river." (2016) The Guardian.