A standard price alert fires once and stops. That is fine for a single target, but it is awkward for a level that price keeps crossing, such as a coin bouncing around a round number.
A repeat alert re-arms when the price crosses back. Say you want to know whenever a coin rises above a level. It fires the first time, then waits until the price drops back below the level before it can fire again. That prevents a stream of messages while price hovers on the line, and it means you hear about each genuine new crossing.
It pairs well with a threshold set outside the normal daily range. If the level is too close to the current price, even a re-arming alert will be noisy.
CoinPriceAlert lists repeat alerts among its alert types on the crypto price alerts page, together with a year of measured daily moves for Bitcoin and other coins, to help you pick a level that is worth reading.