These are activities that allow players to practice skills in an environment that provides external pressures. Closed activities are best for players to learn a skill initially, however they wont progress this skill into game situations unless they can apply these in open activities or skill biased games.
An example of the difference between an open and closed activity could be around a simple 2 v 1 activity. If we had 2 players run at a pole and practice a 2 v 1, it would be a considered a closed activity because there is no defender placing external pressure on the players performing the catch and pass. If we add a defender, instead of a pole, the ball carrier needs to make a decision on if they pass or dummy based on the defender.
Here is an example of an open activity