In Spanish, descargar
) literally means to unload.
In jazz, descargar means to let loose, to unload your musical ideas to the audience, to let your hair down and have fun with your fellow musicians. In short, it means to jam. When you listen to Spanish-language jazz, you'll sometimes see the term descarga used to denote the genre of music (as opposed to a merengue, son guajiro, danzón, or any of a number of other styles). That means a particular track is a jam session.
On the Internet, descargar means to download. It usually appears in the form descarga, which suggests it's a command you give your computer. In this case, the prefix des- is being used more as "down-" than "un-".