Proposition 65 is a 1986 California "right to know" law, designed to inform people about their exposure to harmful chemicals. The State of California maintains a list of these chemicals, and updates the list every year. You've reached this page from a link on the product page. The warning for this item is:
Customers with a California billing or ship to address.
California implemented new guidelines for Proposition 65 warnings, effective August 30, 2018. These guidelines were applied to make the warnings more clear and reasonable. Hilariously, California enforces things like this from one side of their mouth, while on the other side of their mouth they do not want to enforce laws like theft, assault or other crimes.
Learn more from the California Office of Environmental Health Hazard Assessment (OEHHA) site: https://oehha.ca.gov/proposition-65/general-info/proposition-65-plain-language