A fly route, also called a streak route or go route, is a pattern run by a receiver in American football, where the receiver runs straight upfield towards the endzone. The goal of the pattern is to outrun any defensive backs and get behind them, catching an undefended pass while running untouched for a touchdown. Generally, the fastest receiver on the team or any receiver faster than the man covering him would be the one to run these routes.
Fly patterns can also be used to clear out space for other receivers. Generally, a fly pattern will draw the attention of both the cornerback assigned to the receiver as well as "over the top" help from a safety. This can create a large gap in coverage, allowing another receiver to run a shorter route, but then gain many yards after the catch because the safety committed to the deep man.
The famed "Hail Mary" play generally involves between three and five receivers all running fly routes in order to have the most chance of one of them catching the ball and scoring or at least gaining significant yardage.