It means that If your team made the playoffs, That player would not be able to participate in the games that you play because they were put on the transfer list too late in the season. They do this so that teams can't just go out and buy 2 or 3 monsters right before the playoffs to help them win the championship and then sell those players right afterwards. Hope this helped.
For clarification, teams absolutely
can buy 2-3 (or however many they can afford) monsters right before the playoffs. The transfer deadline is to prevent those monsters coming in
during the playoffs.