Antibodies are soldiers deployed by the immune system in response to a foreign invader, in this case SARS-CoV-2. Originally there was hope that antibody tests might enable us to diagnose disease quickly and easily. But it turns out, the tests do not turn positive for two to four weeks (post infection). Even if you test positive for antibodies, that information does not tell you much, apart from the high likelihood that you had contracted Covid-19 in the past.
At this moment, we don't really know what levels of antibodies lead to protection (from disease) and indeed, which kind of antibody is the most important. It is also unclear how long Covid-19 antibodies persist in the body, or that anybody who has tested positive for antibodies will not catch the virus again.
Antibody tests are useful in estimating the spread of Covid-19 at a population level, as well as in measuring the durability of vaccine responses.