New cryptocurrency projects often issue Initial Coin Offerings (ICOs) to fund their new project. The public can buy x percent of the total token supply that was initially offered.
People partaking in ICOs send Ethereum (ETH) to a wallet of a cryptocurrency project and get x number of tokens in return. Most of these tokens are ERC20 tokens because the Ethereum blockchain is most commonly used as a platform to launch these ICO's.
ERC20 is a technical name that's used for smart contracts on the Ethereum blockchain to implement tokens. ERC stands for Ethereum Request for Comment, and 20 is the number that was assigned to this request.
Metal Vault support ERC20 compliant tokens. Make sure to check with the token supplier that the token is indeed ERC20 compliant.
You will have to manually add the custom ERC20 token to your Ethereum wallet in Metal Vault.