I wondered the same thing - As always in cases like this Google is your friend!
A seven stroke roll is a drum roll with an accented (louder? harder?) hit every seventh time, at least thats how I understood it...
Sounds like this:
dak, dak, dak, dak, dak, dak, DAK, dak, dak, dak, dak, dak, dak, DAK, dak, dak, dak, dak, dak, dak, DAK, dak, dak, dak, dak, dak, dak, DAK