Swapping is the act of exchanging one cryptocurrency or token for its equivalent value in another. For example, if you want to buy something that requires DAI but only has ETH to spend, you can exchange your ETH for DAI using the crypto swap service. You can use Bitcoins and other altcoins to swap from one currency to another.
You should note that trading and swapping are not synonymous. When you trade cryptocurrency, you’re executing an order book trade. Only the trading pairs available on the given exchange can be used to execute a trade. While in swapping, you just pay with a different cryptocurrency than the one you want to acquire