jam on

Não sei o contexto em que está inserido mas vamos por partes:

Expressão idiomática: "jam on the brakes" to use the brakes of a road vehicle suddenly and forcefully Eg: A motorbike appeared from nowhere and I had to jam on the brakes.

Jam (as a noun):
#1 a sweet, soft food made by cooking fruit with sugar to preserve it. It is eaten on bread
#2 jam (on a road): traffic jam Eg: We were stuck in a jam for two hours
#3 jam (block): something that is stuck in a machine, or that prevents the parts of a machine from moving Eg: She fed the documents into the machine making sure that there were no paper jams.
#4 jam (informal - difficult situation): I'm in a jam - could you lend me some money till next week? / How are we going to get ourselves out of this jam?
#5 jam (no space): a situation in which a lot of people are in a small space Eg: It's a real jam inside - it took me ten minutes to get to the bar.

Jam (as a verb):
#1 jam (stick): to be, or make something, unable to move Eg: The door jammed behind me and I couldn't get out
to stop radio signals from reaching the people who want to receive them Eg: Foreign radio broadcasts were regularly jammed.
#2 jam (music): to play jazz or rock music with other people informally without planning it or practising together
#3 jam (push): to push something forcefully or with difficulty into something else Eg: He jammed the boxes into the back of the car.
#4 jam (fill): to fill a place completely Eg: The motorway was jammed solid (= the traffic could not move) all morning.

Espero que tenha ajudado.