If something is littered with something it has, or contains, a lot of that thing.
Examples of use:
1. The English language is littered with phrasal verbs.
2. His bedroom floor was littered with books and clothes.
3. This town’s streets are littered with rubbish.
litter (noun) – paper and other rubbish left in a public place.
litter (verb) – to drop litter in a place or to scatter litter in a place.