Precious means dear, important, valuable or irreplaceable, and cargo is the items carried in various containers like ships, trucks and cars. Together the idiom precious cargo has come to mean very dear things that are being carried or moved, and usually refers specifically to children. The sign "baby on board" that appears on many cars is another way of saying the car carries precious cargo.
The term has been used for many other things that have nothing to do with infants or children. There’s even a 2002 Star Trek: Enterprise episode called Precious Cargo which has to do with the Enterprise transporting an ill-mannered but beautiful woman, played by the famous actress and television host, Padma Lakshmi. There are a number of references to the idiom that predate this, and the cargo that is precious may be defined in numerous ways. Anything carried or in the midst of being transported that is of value could fall under this description.
Sometimes when women are pregnant, they also refer to themselves as carrying precious cargo. Being pregnant means acting as a container and transporter for a fetus until birth. It’s more used in a joking way in this sense, and some women may be sensitive to any comparison to large carrying mechanisms like a cargo ship. However, the term, in this sense, is used pretty frequently, as evidenced by the number of maternity stores that bear the name Precious Cargo and are devoted to selling clothing.
There are also many stores that sell items for kids and babies with this name. They may specialize in the types of things that carry and hold kids, like strollers, car seats and the like. It appears that precious cargo has a fairly loose definition that can be adapted for a number of uses, most of them having to do with children.
People may wonder why it is that parents would want to openly signal that they have children present in their car with “baby on board” signs. Probably some parents find these signs important and hope that other drivers seeing them will drive more carefully around them. It’s a reminder to other drivers that there is a child in a car close by, and a request for drivers to be safe around the precious cargo on the road. It’s hard to say whether such a sign works, since auto deaths are still one of the leading causes of infant and child mortality.