A circle with a diagonal line superimposed on the letter E (for estacionamiento) means "no parking." Illegally parked cars are usually either towed or have wheel blocks placed on the tires, which can require a trip to the traffic-police headquarters for payment of a fine.
When in doubt, park in a lot rather than on the street; your car will probably be safer there anyway. There are parking lots in PV at Parque Hidalgo (Av. México at Langarica, Col. 5 de Diciembre), in El Centro (at Av. Juárez at Calle 31 de Octubre and another at Leona Vicario), just north of the Cuale River at el malecón and Calle A. Rodríguez, and in the Zona Romántica at Parque Lázaro Cárdenas. Fees are reasonable—as little as $4 for a day up to $1 or more an hour, depending on where you are. Sometimes you park your own car; more often, you hand the keys to an attendant.