Learn English – What does “wash up” mean in this context

idiomsmeaningmeaning-in-context

Each year as much as 150,000 tons of plastic debris, most notably styrofoam, washes up on the shores of Japan.

What does washes up mean?

This is a tricky phrasal verb, because it has at least three meanings:

to clean one's face and hands : go wash up before dinner
to be have the ocean currents and waves carry something to a beach : after the shipwreck, most of the cargo washed up on the shores of the island
to be past one's prime : _he was a first-rate programmer back in the early 80s, but he hasn't kept up – he's washed up

You can find those meanings (or ones fairly close to that) listed here.

Based on your context, it's the second meaning. The styrofoam is left strewn about on the beach.