Something that happens provisionally is temporary, or depends on certain conditions. Your first driver's license might be given provisionally, on the condition that you only drive during certain times of day.
Sometimes events are scheduled provisionally, like a picnic whose date will change if the weather doesn't cooperate. The adverb provisionally comes from the adjective provisional, "arranged for the present, possibly to be changed later." Both words have a Latin root, provisionem, "a foreseeing, foresight, preparation, or prevention."