A given bike has a fixed voltage, most often 36v; 24v on many older ones and 48v on a few.
Actually the voltage drops a little as the battery is used; eg from 42v down to 30v or so for a nominal 36v battery, but is basically fixed for a given bike.
Current is measured in amps, can be seen as electricity flowing out of the battery. That is likely to go up and down quite a bit as you move from flat to hills etc. A typical value might be 3 amps.
Power at any moment in time is the voltage times the current.
This is measured in watts, which is amps * volts; so you could be using 36*3 = 108w
You can get the same power by more amps fewer volts or more volts fewer amps.
More power will give more speed, and you need (lots) more power for going up hills.
That 108w might get you going at 12mph on a typical slow bike without you helping.
In full power on a hill you may use 15amps and get 540w. (*)
That might get the same bike up a 5% hill (1 in 20) at 8mph without your help.
Total energy used tells how much power you have used times how long you have used it;
so it is measured in wH (watt hours);
If you cycle at the steady power above for three hours you will use 108*3 = 324wH.
In real life you won't be using steady power, but if you add up all the little bits of power minute by minute or second by second it is the total that counts.
Energy comes from the battery, so batter capacity is the energy used in wH.
(apart from inefficiencies in the motor, 80% is typical best efficiency)
So allowing for inefficiencies a 400wH battery will drive you for about three hours at the steady rate above.
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The assist levels typically change the amps that the controller will let the motor use; lower assist is lower amps, lower power, lower speed; but more time before the battery charge runs out.
Gears say how the power gets used so you want lower gears on hills and higher on the flat; but I'm not going into that as well right now. They are separate from the assist levels, but often work best together.; so on a hill you will probably want lower gears and higher assist level.
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numbers above helped by
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(*) even though a legal e-bike has a nominal power of 250w it is common for it to go up to 500w or even quite a bit more for peak power. 250w is the power it can keep up steadily without overheating or other strain.