What are arterioles? What are their functions?

Arterioles are tiny arteries that branch into capillaries.

Their function is for the regulation of blood flow as it arrives into the different body organs / parts. What I mean about regulation is that blood flow in larger arteries tend to have a lot of pressure because bigger bulk of blood is pumped by the heart, the arterioles make the blood flow pressure lower by dividing the bulk of blood into different vessels until it reaches into the tiniest vessels which are the capillaries for gas and nutrient exchange.

For further analogy, think of the larger arteries as 'highways' where there are plenty of 'cars' (red blood cells) and the smaller arterioles as 'streets' where only limited amount of 'cars' can pass through.