Wrapped ETH, or WETH, is a token that represents ETH 1:1 and conforms to the ERC20 token standard. By conforming to ERC20, it allows for increased functionality across any application that handles ERC20 tokens. You can always convert your WETH to ETH 1:1 and vice versa.
In order to use ETH on the Avalanche network, you will first need to wrap it, to WETH, before sending it across the Core.app/bridge.
For any additional questions, please visit our knowledge base or contact a support team member via the chat button at support.avax.network.