Wrapped AVAX or WAVAX, is a token that represents AVAX 1:1 and conforms to the ARC20 token standard. By conforming to ERC20, it allows for increased functionality across any application that handles ERC20 tokens. You can always convert your WAVAX to AVAX 1:1 and vice versa.
In order to use AVAX on the Ethereum network, you will first need to wrap it, to WAVAX, before sending it across the Avalanche Bridge.