What does Montag see in Fahrenheit 451 when he is sitting outside a house watching a telescreen, and what is he thinking?Thanks

I think that you must be talking about what happens when Montag is running away from the Hound.  He has left Faber's house and is on his way to the river.  While he is doing this, he sees scenes of the Hound pursuing him on the parlour walls of the homes he passes.

When Montag sees this, he thinks a few things.  He hopes fervently that the Hound will not stop at Faber's house.  He does not want it to pick up his scent there and either follow him or kill Faber because it thinks Faber is actually Montag.  He also thinks about how this pursuit is being played out on 20 million screens and how crazy it is.  He is drawn to the excitement even though it is his own life that is in the balance.  He finally rejects that (and with it the whole society) and runs to the river and dives in.