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盖中心500毫米以内停车。法定例外要求驾驶员留在车内，或留下持照且能必要时移动车辆的人，不能只因有任何乘客在车内就停车。