The home HIV test works by testing the blood for HIV antibodies -- a positive result means these antibodies have been found in the blood and the person being tested is HIV+. A negative test means there were no antibodies found, therefore the person tested is HIV-.
A person can also get an Indeterminate result which means the test could not confirm or deny the presence of HIV antibodies. This may be due to antibodies just entering the system (meaning the person has been recently infected but too recently to be confirmed on a test) or that they may have another medical condition that is causing the test to fail. It can also mean they are negative.
For this reason, it's important that a person who may have been recently infected abstain from sex until they have a test. Most people have acquired the antibodies in three to four weeks.
https://www.empowher.com/aids-hiv/content/home-hiv-tests-rapid-hiv-tests-and-how-they-work