I don't know how to jam, only jelly:

you see, a jelly session is similar to jamming—except jelly has no substance. -___-

jam ² |dʒøm| |dʒam|

noun

1 a sweet spread or preserve made from fruit and sugar boiled to a thick consistency.

2 (also jam session) an informal gathering of musicians improvising together, esp. in jazz or blues.

jelly |ˈjelē|

noun ( pl. -lies)

a sweet, clear, semisolid, somewhat elastic spread or preserve made from fruit juice and sugar boiled to a thick consistency.