It’s just a formal way of saying “on”. There is no difference in meaning. This is what Oxford dictionaries says about it: The preposition upon has the same core meaning as the preposition on. However, in modern English upon tends to be restricted to more formal contexts or to established phrases and idioms, as in once upon a time and row upon row of seats

OK. I got it. Thank you.

Let’s put Su.Ki.’s comment on my answer up here, so you will be sure to notice it: Or even 'Upon my word!'. The bottom line of this, Valeriy, is that you never need to use the word 'upon'. If you come across it in a set phrase or old-fashioned text, you can just understand it as 'on', and when you are writing or speaking English you can always use 'on'.