The current through a conductor between two points is directly proportional to the voltage across those points, provided temperature and other physical conditions stay constant.
The memory triangle: cover the quantity you want — V = I·R, I = V/R, R = V/I.
V = I × R
V
voltage across the conductor (volts)
I
current through it (amperes)
R
resistance (ohms)
In plain English: Push harder (more volts) and more current flows; add resistance and less flows. Cover the quantity you want in the triangle and what remains is the formula.
Where you'll meet it: Sizing LED resistors, predicting current draw, working out what a mystery resistor is doing — the single most used equation in electronics. Note it only holds for ohmic materials: diodes, LEDs and filament bulbs don't obey it.