The output of an ideal voltage source is stable, and doesn't change when current is drawn from it. The value may also change for other reasons too
Many real-world, practical, voltage sources are not stable and the value of the voltage will change when the current drawn from them changes. Their output may also change if the a.c. supply from which they operate changes, perhaps due to poor design.
A voltage regulator counteracts any voltage change and attempts to keep the value of the voltage supplied at a constant figure.