a current source
A current source is the mirror image of a voltage source: instead of holding the voltage steady, it holds the current steady, and lets the voltage be whatever it must to push that exact current through. If a voltage source is a pump that keeps constant pressure, a current source is a pump that keeps a constant flow rate, raising or lowering its pressure as needed to force the same litres-per-second through whatever you connect.
A 10 mA current source will push 10 mA through its load and adjust its own voltage to do so. Across a 100 ohm load that means 10 mA times 100 ohm = 1 V; swap in a 1 kilohm load and it raises itself to 10 mA times 1000 = 10 V to keep the same 10 mA. An ideal current source would raise its voltage without limit, but a real one runs out of headroom at its supply rail; it can only push so hard before it gives up holding the current.
You rarely buy a current source as a box, but they are everywhere inside circuits. An LED wants a set current, not a set voltage, so a current source, or a resistor approximating one, sets its brightness safely. Transistor amplifiers are biased with current sources, and battery and sensor circuits use them constantly. Recognising that a part wants constant current, not constant voltage, is a real step up in understanding.
To run an LED at a steady 10 mA regardless of small supply changes, a current source beats a plain resistor; across a 2 V LED it simply settles at whatever voltage delivers exactly 10 mA.
A current source fixes the flow and lets the voltage fall where it must.
A current source must always have a path to push its current through; leave its output open with no load and an ideal one would demand infinite voltage, so real current sources hit their rail and stop regulating.