Autosomal DNA is your 22 pairs of non-sex chromosomes. Everyone (both biological males and females) inherits a random mix autosomal DNA from both of your parents (about 50% from your mother and about 50% from your father), and each of your parents inherited their autosomal DNA from both of their parents and so forth. Because autosomal DNA is a mixture of your mother’s and father’s DNA, it is unique to each person and both biological males and females can take this test.
Since autosomal DNA is a random recombination of both parents' autosomal DNA, full siblings will have a different admixture. An analogy would be to say that each parent is a deck of cards. The two decks are shuffled and half of the resulting mixture is taken. One sibling may have more “spades” in their deck while another may have more “clubs.” While each individual has unique DNA, siblings share the same parent “decks,” so we can use the similarities to trace ancestry.
With each generation, the decks are reshuffled, so the “cards” from any given ancestor become fewer and fewer. Thus, the autosomal DNA admixture for any given individual roughly comprises the DNA of all of their ancestors within five generations. This is an estimate, however, as some “cards” may survive many shuffles, while others get “shuffled out” rather quickly.
This is different from our mitochondrial DNA (mtDNA) and Y-chromosome DNA (Y-DNA), which is inherited exclusively through direct maternal or paternal lines, respectively.
Because of the random nature of autosomal DNA inheritance, it is impossible to tell which portions of your DNA came from your mother or your father. However, by comparing your autosomal DNA to that of your known relatives, you can compare results to determine which portions of your DNA come from which ancestral line.