Default account or main account is the account which executes blockchain transactions if none is provided. Most of the functions have executor in the form of payer | accountCreator | contractExecutor.
Local network
When you are on local network eosio account is preset automatically for you as default/main account. You are able to do
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await Account.createFromName('some name')
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without the need of specifying the accountCreator every time.
Another network
If you are on another network, you can set the default/main account as follow: