A slang American term relating to the concept of "secret". It is used as an adjective or a noun, and its origin is from the phrase "down low", as in, hushed, 'low' voices; whispers.
Usage is generally in the phrases "On the DL," or, "This is DL". Sometimes, simply saying "DL" gets across the point that what you are about to say is secret, or saying "that was DL," emphasizes that what you have already said is secret.
The term has a particular further connotation among African-American men who have sex with men, yet do not necessarily self-identify as gay. Among such people, "Down Low", or "DL", is actually the name by which they acknowledge their sexual lifestyle.1 They do not identify as members of a "gay" community. Rather, they are on the "DL" scene, and in fact, they themselves are simply "DL" (rather than gay).
Benoit Denizet-Lewis, a New York Times writer, quoted a self-identified African-American DL as saying,
If you're [a DL who is] masculine and a guy thinks you're checking him out, you can always say: 'Whoa, chill, I ain't checking you out. Look at me. Do I look gay to you?'
As you can see, being an African-American "DL" is not synonymous with being gay.
Obviously, the use of the term in an African-American MSM context is not unrelated to its use in wider culture, but it has additional implications. In general use, "DL" does not necessarily make reference to sexuality, so it's sometimes difficult to figure out what is meant. Context is always the best clue.
1. Denizet-Lewis, Benoit. "Double Lives on the Down Low". The New York Times. 3 Aug 2003.