You need ETH in order to submit any transaction on the Ethereum blockchain. A wallet is where you hold your ETH and how you submit transactions/create/buy/sell items on Mintable.
In order to use a dapp (decentralized application) you need to have a wallet that can connect to dapps. Currently, that would be Metamask, Trust Wallet, Rainbow, and a large array of others.
You can get metamask on as a Chrome or firefox extension. Its super easy to install, it takes about 5 seconds and then about 10 seconds to set up an account.
Get metamask at: https://metamask.io
To load your purchased NFTs into metamask, go here.