How does an underwater gravel filter for fish tanks work?

currently have one it came with the tank but tank has gone ever so cloudy all of a sudden
Answers:
The problem is that your tank is cycling. It sounds like you just set this up and added some fish. The cloudy water is actually pretty common in this situation.

Your fish produce ammonia as part of their wastes. This is toxic to your fish. In a mature tank (and in nature) bacteria convert this into nitrite (which is also toxic), then int nitrate (which your fish can tolerate in a moderate amount). But your tank is new, so you don't have enough of the right kinds of bacteria in it yet. Some are there and they're reproducing as long as ammonia and nitrite (their food sources) are being produced. It's this rapid reproduction by bacteria that you're seeing as cloudy water. It's not anything you're doing wrong, and it's not that the filter isn't working!

You should monitor the ammonia and nitrite so you can do partial water changes when these start to get too high for your fish. You don't want to clean too thoroughly or remove too much water (about 25% once a week is enough unless your water tests show you need it sooner) or you'll set the bacteria back and have cloudy water for even longer. It will clear on it's own in 4-8 weeks usually.

You can avoid buildups under the undergravel filter by using a powerhead (gives better water circulation than an airstone so the filter will work better) that has a reverse flow setting and using the reversed flow to pump wate down the tube and up through the gravel.

---

Under gravel filters are supposed to work by sucking the waste products of the fish down into the gravel. However over time the gravel becomes fully loaded with waste and the filtration stops working.
It also requires that when you clean the filter you need to wash all the gravel.
Better of upgrading and getting a stand alone filter for the tank with a washable filter.

---

an under water gravel filter works by pushing air up and sucking water down like a ventury when the water gets sucked down through the gravel it is filterd but when the gravel starts to get dirty then you need to clean it.