Make and model? It's usually a spring loaded pulley that isn't attached to anything else like the alternator, air conditioner or power steering pump. If the spring doesn't keep the belt tight enough, it may need replacement.

There is an idler pulley with a spring in it that automatically tensions the belt. If the belt is loose, the idler pulley system isn't working - Usually not a big deal to replace.

They vary per motor so make and model would have been good info to know. The shortest way is to look for the pulley that isn't driving anything and is spring loaded.

That's 'Serpentine', (i.e., 'snake-like') and it's a 'tensioner' you want, not a 'tightener'.

Serpentine belt set up has a tension-er, its not adjustable.

we need to know the year, make, model and engine size to answer this one. you may try going online to autozone.com and see if the info is not posted there.