It has several meaning, and its meaning is dependent to the context.
for instance in Music - Jamming is several musicians just playing and messing around on their instruments, not performing. It is usually done to come up with musical ideas.
According to Merriam Webster it means : to fill (a place) completely
: to fill (something, such as a door or a road) so that movement is slow or stopped
: to press or push (an object) into a tight place