It is an example of the passive voice. Here is a similar phrase in the active voice: "For the big dinner, my wife arranged a selection of cheeses on a cheese board." Your sentence, though, is general. We don't know who arranged the cheeses, so we can't say "Lucy arranged a selection of cheeses" or "The host arranged a selection of cheeses." We could create a simple sentence in the passive voice by saying "A selection of cheeses was arranged on a cheese board." For example, "In the banquet room, the table was covered with all kinds of food. A selection of cheeses was arranged on a cheese board." We don't know who arranged the selection, all we know is that somehow, a selection "was arranged." Now, if we wanted to mention the board first and the cheese second, we could change the word order, without changing the meaning much: "On a cheese board, a selection of cheeses was arranged." The next step is that we are adding this onto the phrase, "A cheese board is usually a small tray or board..." We could write this in two sentences: "A cheese board is usually a small tray or board. On this board, a selection of cheeses is arranged." But we want a single sentence. So, we use the words "on which" to join the two sentences and avoid saying "board" twice.

Thanks，Mr.Andrew Tomlinson ！ Thanks for your helping with my English learning although I am still kind of being confused ：）

Right so, in this instance "upon which" means "On top of the previous noun", and it's setting up a second noun to come. Then we have "is arranged", which says that the noun later in the sentence is deliberately placed, usually in an appealing way. Lastly we have "a selection of cheeses." This is the noun that "upon which" is talking about. I didn't submit this as an answer, because I can't explain to you why it's structured that way. It's perfectly acceptable English, but it feels distinctly "High society" to structure it that way. A more common version of this sentence would be: "A cheese board is a board that usually has a selection of cheeses on top of it."