In the television industry, upfronts are meetings held between television networks and advertisers (and, on a tertiary level, the press) to preview what shows will be airing in the upcoming season. The meetings allow advertisers to get a feel for what shows fit with their products and what shows they will want to sink advertising dollars into; they also allow the advertisers to purchase ad time before the season airs, usually at a (slightly) discounted rate.
Calling them meetings is a bit of a disservice, I guess - they usually look something like a cross between the Oscars and the Macworld Expo, with big name executives and production talent. Clips from the shows are usually shown and explicated - the upfronts usually happen towards the end of May sweeps for broadcast networks (and slightly earlier for the cable channels), which is why sweeps are so over-the-top with their explodey, guest-star ridden, death-of-main-character -type action.