What does this marking mean?

Normally do not park within 500 mm of its centre. The exception requires the driver to remain, or a licensed person able to move the vehicle; any passenger alone is insufficient.

一般不得在消防栓或其井蓋中心500毫米以內停車。法定例外要求駕駛員留在車內，或留下持照且能必要時移動車輛的人，不能只因有任何乘客在車內就停車。