That's $400 of net income after expenses (income includes tips). In the case where you make more than $400 gross income, but your expenses offset the income, you may not be required to file. However, it may still be to your advantage to file to get the expenses on the record.
Also, if you file a tax return for any reason, then you must report all your income, even if your net-profit from a side gig is less than $400. The $400 threshold only applies if self-employment is your only income.
