Think of them like this; The Player Pool is a seat of people that could play Soccer, but aren't because there are people who were chosen to be on the normal lineup. Active Roster and Player Pools work the same way. Basically like most sports, you have to swap out these players before being able to use them in the game. Below is a more accurate description:
Player Pool - The Player Pool has a much higher slot limit than the Active Roster, but any players that are in this pool but aren't in the Active Roster are considered Mercenaries. Player Pools are useful to show previous team members, but you can be on more than one Player Pool per season. You can be on a maximum of three team's Player Pools at a time. While in a Player Pool, you can be more conveniently transferred onto another team's active roster, and acts as a way to manage players.
Active Roster - The Active Roster is where your core or substitute players are found. Players on this part of the team are eligible to play all league matches.
Only Players on the Active Roster receive a badge at the end of the season.