The Default address is one address your name gives out for any EVM chain you haven't set a record for. Set it once and every EVM chain resolves to it — Base, Arbitrum, Optimism, Linea and the rest.
Good to know
A record for a specific chain always beats the Default.
Non-EVM chains are not covered. Bitcoin, Solana and the like each need their own record.
How it works with your other records
When a name resolves an address on a chain and returns in this order:
The record for that chain, if you've set one.
The Default, if you haven't.
Nothing, if neither is set. Your name has no address on that chain, so anything sent to it there has nowhere to go.
The ETH Address is Ethereum Mainnet's own chain record. It wins on Ethereum Mainnet the way a Base address wins on Base, and it doesn't answer for other chains. Leave it unset and Ethereum Mainnet falls back to the Default too.
Where you set it
In Edit Profile, the same way as any other address record: add an address and pick Default from the chain list. Full steps are in Add wallet addresses for other chains.
When a Default is the right choice
A Default suits an ordinary wallet, where the same 0x address works on every EVM chain you use.
It suits a smart contract account less well. A contract can sit at a different address on each chain, so those chains want their own records. See Set a primary name for your smart contract.
Your resolver has to support it
Default address resolution is part of the latest Public Resolver. On an older one the Default is ignored, and a chain with no record of its own returns nothing.
Open the name's More tab and look at Resolver. Latest means you're set. Outdated means you're not, so see Update your resolver. Custom means someone set a resolver by hand; whether the Default works depends on it.