Ive been playing for 10 years and i still dont know what it does

Basically, all good gives you hunger and saturation. Saturation decreases as you perform actions, and when it reaches 0, you will start to lose hunger points. The hunger bar will also shake.

Where is it

It's hidden, but basicly all food gives you different saturation and saturation will take food points for you, when you run out it starts taking it hunger points from you.

it's hunger bar when hunger bar too full

If you overfill the hunger bar, saturation 1st heals you until you run out and if you’re at full health it’ll prevent you from losing hunger

are you playing in pc

Yes

then you should know it by the first time playing it "/

what is a saturation bar?