a resistor
A resistor is a component that deliberately resists the flow of current, a controlled narrowing in the pipe. Where a wire lets charge through almost freely, a resistor pinches the flow by a known, chosen amount, turning the leftover push into a little heat. It is the most common part in electronics, the humble workhorse you reach for to set a current, drop a voltage, or protect something delicate.
Resistance is measured in ohms (symbol Ω), and a resistor's whole job is to have a stable, specified ohm value. Through Ohm's law, voltage equals current times resistance (V = I times R), the resistor links the two: put 5 V across a 1 kilohm (1000 ohm) resistor and exactly 5/1000 = 5 mA flows. Resistors come in standard values (the E-series, like 220, 330, 470, 1k) marked by a colour code or printed number, each with a tolerance and a power rating you must not exceed.
Resistors are everywhere because so many jobs amount to letting just this much current through, or dropping a voltage down to a chosen level. A current-limiting resistor protects an LED; a voltage divider of two resistors makes a smaller voltage from a bigger one; a pull-up resistor sets a wire's resting level. They do not store energy or change with frequency; they simply obey Ohm's law and turn excess into heat, which makes them the easiest component to reason about and the right place to start.
To run a 2 V LED at about 10 mA from 5 V, you drop the leftover 3 V across a resistor of R = 3 V / 0.01 A = 300 ohm, so you pick the nearest standard value, 330 ohm.
A series resistor sets an LED's current by dropping the leftover voltage.
A resistor turns the energy it drops into heat, so always check power: a 330 ohm resistor dropping 3 V at 10 mA dissipates only 0.03 W (fine for a 0.25 W part), but bigger voltages or currents can cook it.