Socked on the nose ("SON"), in philately, refers to a cancellation in which the killer, or, if the postmark does not have a separate killer, the (typically circular) postmark itself, is right in the middle of the stamp, within its borders.
Some philatelists and collectors of cancellations have a special interest in, prefer, and/or collect SON mainly due to the fact that the date, time, and place the stamp was used/postmarked can be identified by the cancellation.
This allows specialized collectors to collect, for example, all of the postmarks of a particular country, state, city, county, etc., without collecting entire covers.
Bullseye is another term for socked on the nose. One must note that, bullseye also refers to the earliest stamps of Brazil.