The various cells of the human body are assigned particular tasks. Stomach cells produce acid, kidney cells save water, and liver cells remove certain items from the blood are all examples. Now while each cell is carrying out its assigned function, the DNA contained in its nucleus lives in a very poorly defined form called "chromatin". Special proteins (many provide structural and enzymatic support) are called "histones", and bind to certain areas along the DNA. When the signal to divide and produce new cells is given, this chromatin, along with its histones, begins a cycle of changes becoming a more visible and distinct form called "chromosomes". Except for the ova/sperm cells, there are 22 pairs of chromosomes in each cell called autosomal. These none sexual cells, called "autosomes" , provide the stages for what is called "autosomal DNA". It is the associated histones (to each chromosome) that are marked and analyzed. This analysis becomes the bases for the reports generated.
To give a sense of the relative size of a chromosome, the follow is my attempt to draw in proportion #1 chromosome [an autosome] in relationship to the X and Y chromosomes [the sex chromosomes].
Foundational concept 3 = chromosomes package our DNA including the associated specialized helper proteins called histones.