A white curb can be meant to designate several different things depending on the laws of the city or town, like a passenger load and unload zone or a reserved spot for police or fire department vehicles. White curbs often designate tow-away zones where people are only permitted to park for a few minutes or not at all.
A white curb can have different meanings depending on the geographic setting. In the city of Seattle, for instance, a white curb could mean that the area is a passenger unloading and loading zone. This simply means that people are only allowed to park in the area for a brief amount of time for the sole purpose of dropping off passengers or picking them up. They can't load or unload furniture or any other kinds of items. Furthermore, they are only allowed to park there for a few minutes. These kinds of white curbs often show up in locations where many businesses and residential areas are nearby.
White curbs can also be used to designate area where only police and fire department-related vehicles are allowed to park. These types of areas can be designated tow-away zones where if a civilian parks their vehicle there, they may suffer a ticket and/or having their car towed.