1. Sometimes abbreviated as dl or DLing, download is a term used to describe the process of copying data from another computer, either over a network or modem. For example, each time you visit a web page on the Internet, you download the information on the page, including any pictures, to your computer. The term download is often associated with pictures, songs, videos, and programs. A good example of this is downloading a video from YouTube so that you can save it to your hard drive.
The speed at which your computer downloads a file depends largely on the speed of your Internet or network connection. A faster Internet or network connection typically results in a shorter amount of time to download a file.
2. DL is also chat slang for down low. For example, "Keep it on the dl."