You already know two quantities of a circuit: the current (in amperes) that flows, and the potential difference (in volts) that pushes it. Today we connect them. The whole point is one relationship — and the honest limits of when you are allowed to use it.
1. Present the physical scene
Picture a single torch cell, a length of nichrome wire (the dull grey wire used in heaters), and an ammeter to read the current. You connect the cell across the wire. The ammeter shows some current — say a small, steady reading.
Now you swap in a second identical cell, so the push is twice as strong (two cells in a line give twice the potential difference). You watch the ammeter.
Stop scrolling. Try it in your head before reading on. With twice the push across the same wire, what does the ammeter read now — about the same, a bit more, or roughly double?
(Answer: roughly double. Two cells give twice the potential difference across the wire, and the current jumps to about twice what it was. Push twice as hard, get twice the flow. Try three cells and the current is about three times the original. The flow follows the push, step for step.)
You can now name the scene we are about to explain: across one fixed wire, the current rises in lock-step with the push you apply.