This is a phrase used in football to refer to a list of players to be used made by a coach for the upcoming game. This is not an official term. The idea of making the list is to determine which of a team's players have the most potential to overcome the opposition. It usually consists of a team's most skilled and athletic players. The coach will attempt to identify the strengths and weaknesses of the opponent and find players on their own roster capable of overcoming them.
In the NFL, a ready list is usually made by individual coaches within a specific section of the team. The head coach may also distribute an ideal list which is then expanded upon by other coaches. This is because teams typically have one position coach with many assistants in addition to the head coach and offensive and defensive coordinators. Making one list would be difficult, so coaches will make their own and combine later, if necessary.