It is not usually referred to as the Rover anymore, as defenses have evolved. In past years (several decades ago) a rover was a strong safety in a 5 scheme ( 5 man line) -- which made him a hybrid linebacker/defensive back.
For example, in a 5-2 Monster Defense, the rover was the "monster". You had a 5 man line and 2 LBs, and the rover/strong safety/monster (or "The Wolfman" as he was known in Joe Paterno's defense of the late 60's/early 70's at Penn State) might line up on the offense's strong side, or on the wide side of the field, or in a position to "key" on the offenses top performer and go wherever he went.
In an older version of the 4-3 defense, the Rover was generally a defensive back who would rotate in a zone to come up and support on runing plays and cover the flats on passing plays.
However, in the 4-3, while the rover was a DB, the Monster was the Middle Linebacker.
You can still find a "rover" at the high school & small-college level, but not much any longer in major colleges.
Hope this answers your question.
24 years of H.S. football coaching. Much longer as a college and pro football fan.