Pastoral provision is like a special rule for certain people who want to be part of the Catholic Church, but they can't follow all the ordinary rules because they're married and their husband or wife is not Catholic.
So, the special rule is that these people can still belong to the Catholic Church and be priests, but they can't be married. This is because in the Catholic Church, priests aren't allowed to get married. But, if someone was already married before they became a Catholic, they can still become a priest as long as they follow the special pastoral provision.
It's kind of like when you have a special way of doing something because of a special circumstance. For example, if you have a peanut allergy, you might have a special rule of not eating peanut butter. In the same way, if someone wants to be a priest but can't follow the regular rule of not being married, they can still be a priest through the pastoral provision.