Compound In A DLookUp Function?

In these three formulas for the Ceil tutorial found here:

[URL]

one of the formulas is:

=IIf(IsNull([RoomNumber]),"",DLookUp("RoomType","Rooms","RoomNumber= '" & [RoomNumber] & "'"))

If one were interested in BedType or Rate then simply substitute that term for RoomType.

Now I am somewhat familiar with how DLookUp works. In other words get RoomType from a table Rooms where there is a given RoomNumber.

Where I get confused is the last clause:

"RoomNumber = '" & [RoomNumber] & ""

I realize it is a compound statement hence the &, but it uses two &s and more.