Plastic in short
We know what fossil fuels are. They are remains of plants and animals compressed over a period of millions and millions of years. They come in 3 forms: crude oil, natural gas, and coal.
Out of this, crude oil is mainly built out of marine life, settled on the ocean floors and compressed over million years. We get various things out of crude oil and one of those is 'petrochemicals'. When we chemically rearrange this for long term stability, we get polymers (plastic).
Plastic is basically ancient marine flora and fauna turned into a water bottle. Of course, we didn't plan it properly and now it is everywhere where it shouldn't be - aka causing pollution.