It's a matter of supply and demand.

Basically amperes (amps, A) is just a unit of electrical current, it's measuring the number of coulombs worth of electrons that pass through a circuit in a second. Multiply that by a unit of time, such as hours, and you've effectively got a count of the number of electrons that the battery is theoretically capable of pumping through your device before it's expended and needs recharging.

So if you ran your tablet from a battery for an hour, it would consume two amp-hours. Smaller batteries are often measured in terms of milli-amp-hours or mAh, milli being the SI prefix for one thousandth. So 2Ah = 2000mAh.

It's worth pointing out two facts here:
- The 2A is likely to be a peak rating and that depending on what your tablet is doing, its overall consumption could be less than this.
- The rating of the battery will be impacted by a lot of factors and is at best a theoretical rating. As it becomes depleted, its output voltage will drop, possibly to levels below what is needed to power the tablet, so you may not be able to use all the theoretical capacity.

As for your dynamo… its generating current by your movement down the road. If for part of your journey, you're slugging it up hill, it's possible that your generated output is something less than the 2A needed by the tablet, so your battery has to pick up the slack, it's a net drain on the battery. Suppose you were able to muster about 300mA, the battery has to supply the remaining 1.7A.

Then you get over your hill, suddenly the generator is pumping in 4A. Supply is now in surplus so you'll be keeping the tablet alive and pumping up to 2A back into the battery.