Resistance tells you how hard it is for charge to flow through a component. The potential difference (V, in volts) across a component equals the current (I, in amperes) through it multiplied by its resistance (R, in ohms). In symbols, V = I × R.
The equation can be rearranged. To find current, I = V ÷ R. To find resistance, R = V ÷ I. For example, a current of 3.0 A in a 4.0 Ω resistor needs a potential difference of V = 3.0 × 4.0 = 12 V. If a 12 V supply is put across a 4 Ω resistor, the current is I = 12 ÷ 4 = 3 A.
For the same potential difference, a larger resistance means a smaller current. If the resistance doubles, the current halves. A 6.0 V cell across a 12 Ω resistor gives I = 6.0 ÷ 12 = 0.5 A. A 12 V supply that drives 3 A has R = 12 ÷ 3 = 4 Ω. With the same resistor, 24 V would give 6 A.