We often use on for surfaces when talking about physical spaces and days when talking about time.
On can be used as a preposition in the following situations
Physically in contact with, supported by something, or cover something
We use on for situations where one object touches another object and the first object supports the second object. This can be in any direction( from the floor, from the wall, from the ceiling). We also use it when we talk about technology because a lot of technology uses comes in the form of a surface of a screen.
I sat down on the chair.
I put the cup on the kitchen counter.
I watched the movie on T.V.
I study English on Youtube
I put the painting on the wall.
I put the paint on the canvas.
He has a scar on his leg.
You need to put your coat on.
Put the tinfoil on the pork chops.
Express the medium through which something happens.
I called her on the telephone.
I payed for it on my credit card.
She was drunk on wine.
We also use on when we want to express the current state of a process.
She put me on hold.
That argument is on point.
He is on episode 5 of the T.V series.
There are other informal uses of on that is used as a preposition including the following
The dinner is on me.(paid for by me)
I have no money on me.(I have no money with me now)
ON as an adverb
On can also be used as an adverb. It is less common to use as an adverb. Remember, an adverb is a word used to modify(change the meaning) a verb.