A Second Striker, a Deep-lying forward or a Sub forward is a striker whose roles is to create chances and scoring opportunities for the other/main striker in their team. Second strikers usually play just in behind the main striker.
A second striker is usually able to dribble and pass the ball very well. Depending on the abilities of the main striker, the second strikers are able to cover the weaknesses of the main strikers. For example, if the main striker is not good enough at header, usually a second striker who is good at header is positioned behind him to cover the issue.
Centre forwards and Central attacking midfielders with high attacking work rates can be also used as second strikers.