When they say 15C discharge rate, what are they talking about?
They talk about nonsense.
C is the battery's capacity in Ah.
15C is meant to be the current that will fully discharge it (which is not really posssible) in 1/15 hours.
E.g. 2200mAh capacity: 15C = 33A
To be more clear, this is what manufacturers state their batteries are capable of discharging at. 15 times the capacity.
This is most known in the remote control hobby world. The powerful electric motors eat up a lot of power and need high discharge rates. It is not nonsense. If you don't have a battery capable of the discharge rate the motor demands, it can do 2 things. Slow the motor down and make it sluggish when accelerating or being put under a load. It can also kill the battery in as short as a few cycles. With a LiPO battery this can end up with a not so nice result of setting everything on fire.
E.g. 2200mAh capacity: 15C = 33A
Think of it as the manufactures rating for how much peak current can safely be drawn for a short period. It does not mean you can expect to draw the full 2200mah at 33A, it will be less at that discharge rate.
Lefty
It is not nonsense
Of course it is nonsense!
Battery capacity and discharge current do not neccessarily have anything to do with one another. They sure become proportional when combining batteries in parallel. Otherwise high discharge current is a criteria of battery technology. When using small and light batteries they will have of course a higher "C-factor" than high capacity batteries, although this is not proportional.
A moreserious value would be the permanent allowed current flow (upto the safe discharge voltage), and the peak current. You find those values in the datasheets if you buy a high quality battery.
For low price batteries a major issue seems to be at the moment that the capacities given are not reliable. Many of them have half of the announced capacity only. So multiplying things might give you the wrong impression....