first the spring puts tension on the belt, then you tighten the tensioner assembly and it hold there.
it is a spring loaded pulley that keeps a constant pressure on the belt
A spring loaded pully that keeps the serpentine belt tight
You did not list Make, Model, Year, or Engine Size. So all I can tell you is that the vast majority of vehicles with a timing chain have a self tensioner that keeps the tension tight. If the chain it loose or noisy then it is more than likely worn out or the tensioner needs replacing or both.
The belt tension should adjust automatically. There's a thing called a belt tensioner that has a strong spring in it (you can't see the spring) that keeps pressure applied to the belt. This belt tensioner is what makes the serpentine belt so hard to remove. Many times the tensioner is in a difficult place access.
There is a spring-loaded tensioner pulley that keeps tension on the belt. The tensioner should have a place to fit a ratchet wrench or breaker bar (may or may not need a socket). The tensioner bar rotates clockwise or counterclockwise to release the tension on the belt. Be sure and note how the belt is routed so you will install the new belt in the same pattern. The tensioner is self-adjusting.
if your belt is loose, the tensioner probably needs replacing. the tensioner is the mechanism that keeps the belt tight.
The serpentine belt runs from the crankshaft pulley to all the units that need to be turned, alternator, power steering, water pump, etc. There is one pulley that is spring loaded that keeps the correct tension on the belt. That is the tensioner pulley. That can be moved to release the tension on the belt so that the belt can be removed.
It keeps drive belts under tension (pulled tight)
Your Explorer has a serpentine belt tensioner which keeps the proper tension on the serpentine belt unless the tensioner has failed.
If I understand your question correctly, you are asking about a pulley / drive belt tensioner. The drive belt turns the alternator and other auxiliary equipment (sometimes power steering pump) from the crankshaft pulley. The belt tensioner is fitted adjacent to the belt and keeps it under tension automatically by use of a spring. Looking under the bonnet from the front of the car the drive belt and alternator are to the left hand side of the engine.
You do not tighten a serpentine belt. It has a tensioner that keeps it at the correct tension. If it is loose, either the belt is worn and needs replacing or the tensioner is defective.
On the front of the engine. It keeps the belt tight.