1. Jam jar rhymes with car. So jam jar means car. A private motor vehicle. The full phrase , jam jar, is always used. This phrase conjures up visions of a ford escort with the names Trevor and Sharon written on the windscreen. This is classic Cockney rhyming slang. It is necessary to use the whole phrase "jam jar", it is not possible to use jam on its own.
For example: I crashed my jam jar into a wall last night.
I haven't got any tax, MOT or insurance for my jam jar.
He is looking for a new jam jar because his present one is an old banger.
2. In normal speech a cylindrical glass container for jam. When you buy a jar of jam it comes in a jam jar.
For example: I dropped a jam jar and it smasshe on the floor.
I'll put these flowers in this jam jar with some water.