Blockchain transactions are not instantaneous: their execution may take from a few seconds to over an hour depending, primarily, on which network and the number of transactions users try to execute at a given time.
At the same time, cryptocurrency is volatile and the prices of coins fluctuate constantly, sometimes in a matter of minutes.
As a result, when you swap one coin for another one it is common that the prices when the order actually executes are different than the prices the coins had when you confirmed and submitted a swap.
Zelus wallet highlights this by indicating the “Minimum amount you’ll receive” before you confirm a swap. The swap will only take place if and when this minimum amount can be guaranteed, preventing you from swapping a coin at an undesirably low price.