According to the CDC, if you test negative for COVID-19, that means the virus that causes COVID-19 was not found in your specimen. If you are experiencing COVID-19 symptoms and test negative, your current illness is likely not COVID-19, though it does not completely rule out a COVID-19 infection. In this case, the CDC recommends that you test again within 2-3 days and isolate until you retest.
A false negative is also a possible outcome for a negative test result. You may also get a false negative if the specimen was collected too early in your infection. If that is the case, you could test positive later during your illness.