a voltage divider
A voltage divider is two resistors stacked in series across a voltage, with a wire tapped off the junction between them. It hands you a chosen fraction of the input voltage. Think of a ladder leaning against a wall: the top is the full supply, the bottom is ground, and the junction is a rung you grab partway up to get part of the way to the top. It is probably the most common little sub-circuit in all of electronics.
The output is Vout = Vin times R2 / (R1 + R2), where R2 is the lower resistor, the one between the tap and ground. The fraction is simply the lower resistor's share of the total. For example, a 10 kohm on top and a 5 kohm on the bottom across 9 V give Vout = 9 times 5 / (10 + 5) = 3 V. Two equal resistors give exactly half the input. Swapping the two resistors changes which fraction you tap, so the divider is easy to tune just by choosing the ratio.
Designers reach for dividers daily: to set a transistor's bias point, to make a reference voltage, or to scale a too-big signal down into the range an analog-to-digital converter can read. But there is a crucial honesty here. That clean formula assumes nothing is drawing current from the tap. Connect a real load and it pulls the tap voltage down, an effect called loading. The fix is to make the divider's own current much larger than the load current, or to follow the divider with a buffer.
To get 3.3 V from a 5 V rail for a reference, pick R1 = 1 kohm on top and R2 = 1.8 kohm on the bottom: Vout = 5 times 1800 / (1000 + 1800) = 3.21 V, close enough, and only if the next stage draws almost no current.
The lower resistor's share of the total sets the tap voltage, if the tap is lightly loaded.
The simple divider formula only holds when the load draws negligible current. Connect a real load and the output sags; size the divider current well above the load current, or add a buffer.