A provisional ball is a backup you hit before you go look for the first ball, when the first one might be lost or out of bounds. You must say the word "provisional" out loud before you hit it, or it simply becomes your ball in play with a penalty.
When can I play a provisional ball?
Play one when your ball might be lost outside a penalty area or might be out of bounds. It is also allowed when the ball might be in a penalty area but could also be lost somewhere else. You don't need one when you already know the ball is in a penalty area. That has its own relief.
What do I have to say?
Tell someone in your group before you hit that you are playing a provisional ball. Saying "I'll hit another" or "I'm reloading" is not enough. Use the word "provisional" or make it clear you mean that. If nobody is close enough to hear you, play it and tell someone as soon as you can.
If you skip the announcement and hit again from the same spot, that second ball is your ball in play under stroke and distance, with the penalty stroke.
When does the provisional ball become my ball in play?
It takes over if your original ball is lost outside a penalty area or is out of bounds. You then count the penalty stroke and carry on with the provisional ball. It also takes over if you hit it from a spot nearer the hole than where your original ball is estimated to be. After that, your original ball is out of play even if you find it.
When do I have to give up the provisional ball?
If you find your original ball on the course within three minutes of starting the search, you play it and you abandon the provisional ball. Strokes you made with the provisional do not count. If the original is found in a penalty area, you either play it as it lies or take penalty-area relief.
You can keep hitting the provisional ball and it stays provisional, as long as each shot is from a spot at least as far from the hole as where the original ball is estimated to be.
The governing rule is Rule 18.3.
Local rules
Your league's committee may have local rules that change this. Ask the organizer which ones your league uses.