Some rules will require you to take Holds. These Holds allow you to take actions out of turn, above and beyond the limit of your Reactions. Any such rule will indicate the number of Holds that you receive, as well as a condition under which a Hold may be used, and a list of options available when using a Hold.
When you receive any Holds, they replace any Holds that you may have. You may choose to retain any Holds that you have, and not receive new Holds, but any Holds that you have can only be used under the conditions outlined by the rule that granted you the Hold, and can only be used to select actions described by that same rule.
As an example, Cerwyn uses The Game's Afoot, and receives five Holds. She later decides that she needs to Stand In Defence of Miken, and gains two Holds. She loses the five Holds she gained from The Game's Afoot, and she can only use the three Holds she gained from Stand In Defence as outlined in that action.
So long as you have even a single Hold, you lose your Reaction, but can always take a free Reaction to use a Hold. However, you may never spend more than one Hold per provoking action.
Having Stood in Defence of Miken, Cerywn receives two Holds. When an Orc attacks Miken, she can choose to spend a Hold to impose slight on the attack roll, or to take the attack herself. She cannot do both, as this is a single provoking action.