I heard that word several times when i was listening to "Underdog Project - Summer Jam" but till now i don't know what the word mean....and i'm very curiously to find out...
I presume that it is just a 'jam session'-- a group of musicians improvising-- in the summer season. Google seems to bear me out, talking of:

'This ain't nothing but a summer jam
Bronze skin and cinnamon dance whoa!
This ain't nothing but a summer jam
We're gonna party as much as we can...

Tonight hotties wearing "Prada"-skirts
Real tight temperature is rising
Feelin' real hot in the heat of the night...'

summer jam mean summer hit