From Reason:

The phrase “Second Amendment sanctuary” is an umbrella term used to describe a jurisdiction that passes a resolution declaring that restrictive gun control laws another legislative body passes are unconstitutional and will not be enforced there. The concept is an adaptation of the immigration “sanctuary city” movement, in which some cities and counties (and now whole states) generally decline to ask residents about their immigration status or assist the federal government in enforcing immigration laws. The resolutions can vary, but generally, Second Amendment sanctuaries refuse to dedicate resources to enforcing things like “red flag” laws and bans on certain weapons.