The internal resistance is a measure of how well the cells can deliver current.
The lower the internal resistance, the more current you can draw from the cell without losing voltage.
Most NiMH cells are relatively high internal resistance when compared to other types, such as lithium packs of an equivalent capacity.
That's why I use A123 (LiFePO4) packs for my receivers/servos. They have a very low internal resistance and even the 1100mAH packs can deliver over 30A continuous and 50A for a few seconds. There's no way you could get that much current out of a 1100mAH NiMH pack.
Why is that important for a receiver pack?
Well although most of the time a receiver and servos are only going to be drawing a tenth of an amp or two, there are times (such as when you command several servos to move at once) that the instantaneous current draw can surge to several amps or more. This is because when the servo first starts to move -- the inertia of the motor, gears pushrods and control surfaces all work to place a very heavy load on it -- almost as if it were stalled.
If your receiver pack has a high internal resistance then the sudden load of several servos staring up simultaneously can cause the voltage to drop so low as to cause problems with the receiver (especially if you're running a 4-cell pack with a DSM2 system).