It's a setting that, when turned on, transfers out all of the cash from dividends and interest to your bank account. When turned off, the cash from the dividends and interest goes into the cash component of the portfolio and then we use it to buy more stocks. This is like our version of a reinvestment plan.
If you have the income sweep turned off, then any distributions and dividends will remain within your investment account. It will therefore be available to purchase additional holdings across the portfolio whenever it is deemed that a rebalance of your account is necessary.
If you have the income sweep turned on, then any distributions received during a calendar month will be accrued and paid out in the following calendar month as per the above.
You can check or change your income sweep settings under Portfolio Actions > Investment Preferences or here.