A post is a moderate to deep passing route in American football in which a receiver runs 7-10 yards from the line of scrimmage straight down the field, then cuts toward the middle of the field (towards the facing goalposts, hence the name) at a 45 degree angle.
It is designed to stretch the opposing secondary deep up the field, exposing holes underneath the coverage over the middle. It works particularly well against secondaries that don't have more than one safety who are effective in coverage. If a post can be run effectively it will force an opposing defense to play a deeper field and drop more defenders into deep coverage, which opens up the run. Cover 3 packages can be effective against it, if the defender in middle deep coverage is perceptive enough.
To run the route truly effectively, a wide receiver is required to be adept at catching the ball in traffic, and to have the vertical ability and strength to be able to go over top of a safety to get the ball. This is why Randy Moss was able to master the skinny post. Mushin Muhammed isn't bad at it either.
A variant of the post pattern, in which the receiver cuts infield at a shallower angle, is called a skinny post. It is designed to find a hole in deep coverage, cutting shallow inside the deep sideline defender, but not far enough to draw the middle defender.