Memes can be considered the unit of cultural evolution. Ideas can evolve in a way analogous to biological evolution. Some ideas survive better than others; ideas can mutate through, for example, misunderstandings; and two ideas can recombine to produce a new idea involving elements of each parent idea.
The term is used especially in the phrase "meme complex" denoting a group of mutually supporting memes that form an organised belief system, such as a religion. However, "meme" is often misused to mean "meme complex".
Use of the term connotes acceptance of the idea that in humans (and presumably other tool- and language-using sophonts) cultural evolution by selection of adaptive ideas has become more important than biological evolution by selection of hereditary traits. Hackers find this idea congenial for tolerably obvious reasons.