It is common practice for decentralized exchanges to ask for an approval as a first step before executing a swap. This is because decentralized exchanges use smart contracts to execute swaps in a safe manner and, by design, these smart contracts can’t access your crypto unless you approve access from your end.
In these cases, swaps require 2 steps, two separate orders to be recorded in the blockchain. The miners of the network are compensated with a gas fee (paid by you) to record each of these actions on the blockchain.
First you approve the smart contract to move your crypto (requires a small gas fee)
Second, swap the coins (requires a relatively higher gas fee)
This is only required the first time you interact with a specific decentralized exchange: once you have successfully approved your coins to be used with a contract you won't need to approve them again.
In Zelus wallet, when you choose to swap two coins that require this type of preliminary permission you will be shown the details of the approval, as illustrated in the screenshot below. Tap on ‘Approve access’ and wait a few seconds for the approval to be confirmed.
Once the approval has been confirmed the ‘Swap details’ card will expand and the action button will change to ‘Preview swap’. Tap this button to continue the swap.