What does the phrase ‘a high line’ mean? this posts explains this footballing terminology.
This phrase refers to a style of play which pushes the defenders high up the pitch, near the halfway line. It is usually used to press the opposition, to put pressure on their players when they have the ball. This is particularly effective against teams that like to keep possession and pass the ball a lot. The negative side of playing a high line is that there is a lot of space behind the defenders, meaning the team is vulnerable to the counter attack.
Example: Villas-Boas enjoyed great success with a high line at Porto – although even there, the high defensive line seemed very risky at times.” (zonalmarking.com)