Reconciling your Gnucash account simply means you are matching the income and expense transactions you entered into Gnucash against your business banking checking account for accuracy which is very important that everything you enter into Gnucash balances with your banking statements.
Double-checking, your records against your banking account transactions each month will give you a better understanding of your transactions such as uncashed checks and undeposited amounts owed to you from third parties.
Gnucash can help you catch any discrepancies between your Gnucash business transactions data and your business banking account monthly statements.