A Trailing Stop in both MT4 and cTrader is essentially a Stop Loss that automatically gets modified (changes the SL price) as the market price rises for Buy Orders or decreases for Sell Orders.
The way it works is simpler than it probably sounds.
Let’s assume that the price of EURUSD is 1.20010 and that you opened a Buy Order at that price.
Adding a Trailing Stop of 10 Points will automatically add a SL on your position at 1.20010 once the market reaches the level of 1.20020
(1.20020 - 1.20010) = 10 points or 1 pip
If the market goes even higher at 1.20030 then the SL will automatically be modified at 1.20020 and so on.
While the market drops, the SL will not adjust or readjust itself. That only happens while the market is moving at your order’s direction.
NOTES: Not available on Skilling Trader though - if set - via cTrader it will be reflected on Skilling Trader as well.