Say it with me, out loud, right now, "Voltage is current times resistance". That's Ohm's Law.

Voltage = Current * Resistance

Voltage is in volts (V).
Current is in amps (A).
Resistance is in ohms (Ω).

You'll also see it shorted to "V=IR". Voltage is V and resistance is R, obviously. Current is the letter I because it was originally called "Intensity of current".

We'll now do one basic example.

1 amp flowing through a circuit means about 6.25 * 10¹⁸ electrons are flowing through a point in that circuit in one second. 6,250,000,000,000,000,000 electrons.

So in the circuit above, 0.009A, or 9mA, is flowing through any point in the circuit. So if you do the math - 6.25 * 10¹⁸ * 0.009 - you'll find there's about 5.6 * 10¹⁶ electrons per second flowing through any given point in the circuit in the video above. 56,000,000,000,000,000 electrons.

Eventually there will be circuits with different amounts of current flowing through different parts of the circuit, but this is all you need to understand for now.

How exactly is this useful in circuits? The most immediately important application of Ohm's Law is actually safety. As you're about to see in the next section on power, more current means more power. And more power means more heat. Too much power means too much heat. Too much heat means your circuits... get extremely hot. So hot that they damage themselves, and can potentially burn you in you touch them. As you're about to see, before you can even dig into designing basic ciruits, you have to determine which parts are safe to use.