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.
When you are on local network eosio account is preset automatically for you as default/main account. You are able to do
await Account.createFromName('some name')
without the need of specifying the accountCreator every time.
If you are on another network, you can set the default/main account as follow:
const eoslime = require('jungle').init();
const jungleMainAccount = eoslime.Account.load('name', 'privateKey');
eoslime.Provider.defaultAccount = jungleMainAccount;
In this way jungleMainAccount will execute every blockchain transaction whenever the execution account is needed but none was provided.