Prevalence means there is a lot of something. If you get a bad grade on a paper for having a prevalence of typos, you need to proofread your work more carefully.
The word prevalence comes from the Latin praevalere, meaning "condition of being widespread or general." The word is often used to describe a phenomenon that is widespread in a community, like the prevalence of a disease across a country. Prevalence is another word for "commonness." If there's a prevalence of thefts in your neighborhood, that means the likelihood of your house getting robbed is pretty high.