A trailing stop is a type of order that is similar to a stop order, except that it will automatically move in the direction of the trade. Namely, up for long trades, down for short trades. It will not update the stop if the price starts moving against you.

A trailing stop can be replicated manually by updating your stop losses as the trade goes in your favour.

For example, say ETH is trending upwards, so you go long on ETH at $200, and you place a stop loss at $190, that way if the trend reverses you'll only lose 5%. Then the price goes up to $225, and you want to move your stop loss to $210 so that instead of at worst losing 5%, you're at  worst gain 5%. You then do the same when the price hits $250, $275, $300, etc.

That'd be a lot of manual effort, and checking of your account, and more error prone because the price of an asset may shoot up when you're not looking, and crash down even harder before you have a chance to check. The trailing stop continuously moves the stop in the direction of the trade as the prices moves.

With a trailing stop however, you set the amount you want the stop loss to trail behind the best price, and then can walk away and forget about it. You can set the trailing stop in either a percentage (5% below the price) or an absolute ($10 below the price).

So for example, you go short on ETH at $200 with a 5% trailing stop. When ETH moves down to $180, your stop loss would be updated to be $189 automatically, [$180 + ($180 * 5%)].

Trailing stops are really good in trending markets, and when you don't have time to watch the trade yourself.