What IS a .fits file, as in, what is a .fits array?
A FITS file consists of header-data units. A header-data unit contains an ASCII-type header with
keyword-value-comment triples plus either binary FITS tables or (hyperdimensional) image cubes.
Each entry in a table of a binary FITS table may itself contain hyperdimensional image cubes. An array
is some slice through some dimensions of any of these cubes.
Now as a shortcut to images stored in the first (a.k.a primary) header-data unit, many viewers
allow to indicate in square brackets some indices of windows into these images (which in most
common cases is based on the equivalent support by the cfitsio library).
I'm basically trying to plot some images based on a given set of parameters of a .fits file. However, this made me curious: what IS a .fits array? When I type in img[2400,3456] or some random values in the array, I get some output. 
I guess my question is more conceptual than code-based, but, it boils down to this: what IS a .fits file, and what do the arrays and the outputs represent?