California's Safe Drinking Water and Toxic Enforcement Act of 1986, better known as Proposition 65 (Prop 65), was enacted to address California consumers' concerns over possible exposure to toxic chemicals. Prop 65 requires the State of California to maintain a list of chemicals believed by the State to cause cancer or reproductive harm at certain levels of exposure. That list has grown to around 900 substances.
If a product contains any of these chemicals, a warning label may be required. Note that Prop 65 is a "right to know" law that advises if certain chemicals are present; it does not ban or prohibit use of any of the 900 listed chemicals. Many of the chemicals on the list (such as Vitamin A, testosterone and estrogen) are necessary to human life but can have deleterious effects at certain exposure levels.
This is not a disclosure that these products present a threat to users' health or that these products are not designed to meet or exceed all applicable laws regulating product safety and content. This includes the internal Environmental Quality Standard which is designed to satisfy the European Union's Restriction of Hazardous Substances (RoHS) Directive, which applies specifically to the electronics industry.
To learn more about Prop 65, please visit the website of the California Office of Environmental Health Hazard Assessment (OEHHA): https://oehha.ca.gov/proposition-65.