A trial balloon is information sent out in order to judge reaction on the part an audience. It can be used by companies sending out press releases to judge reaction by customers, or it can be used by politicians who deliberately leak information on a policy change under consideration.
For instance, a company might announce they are going to release a new computer program in a year, and then read the press coverage for hints on whether or not the product will have appeal in the marketplace. If the coverage is favourable the money will be spent on development, but if not the project can be cancelled before using up resources. A trial balloon is somewhat risky, if too many are "floated" the company risks becoming know as the company that cried wolf, and can find itself being ignored completely. In addition, the company can find that the product being planned is unworkable, leading to the phenonmenon of vaporware.
One concrete example was Boeing's announcement of several new airline designs in the wake of the Airbus A380 entering development. First they offered a similar-sized large aircraft, but with the Airbus A380 beating them to production by at least two years, there was apparently little market left for Boeing. They then announced that they had cancelled their large airliner planes and announced the Sonic Cruiser design, and found much the same reception. This led to the Boeing 7E7 Dreamliner as yet another offering looking for a market, one that appears to have found interesting customers.
