A Landlord Gas Certificate is a certificate that every landlord or commercial property owner needs to obtain every year, if their property has any gas appliances. You need to have a certificate for each gas appliance in the property.
The certificate confirms that the appliance (boiler, gas hob etc.) has been checked and is safe for your tenants to use.
CP12 is an old name for a Landlord Gas Safety Certificate. While it's not the official name for this kind of certificate anymore, lots of people still refer to them as CP12s.