What does it mean when your Vanilla Prepaid Mastercard balance is on hold?

just what it says. A hold means the money is not available. But it may or may not be in the future.

Lets say you buy something online for $67.24. And then cancel an hour later. The $67.24 will be on hold for 3-5+ days until they process the refund.

Possibly you have hired a car using the card and the hire company have 'ring fenced' a sum of money against a deposit or the like. You will eventually get access to the money providing there isn't any claim they make against that money,

means you cant use it