A self-report measure is an assessment in which people answer questions about their own thoughts, feelings, or behaviors, rather than being observed or rated by someone else.
Most personality tests, including the vast majority of Big Five assessments, are self-report: you're reading statements like "I see myself as someone who is talkative" and rating how much they describe you. This approach has real strengths — nobody has more access to your internal thoughts and typical behavior across situations than you do — but it also has known weaknesses. People can be biased toward presenting themselves favorably (social desirability bias), or they may simply lack accurate self-insight into their own patterns.
The main alternative is informant report, where someone who knows you well — a partner, close friend, or coworker — rates you instead. Interestingly, self- and informant-reports tend to agree reasonably well for traits that are easy to observe, like Extraversion, and agree less for traits that are more internal, like some aspects of Neuroticism.
Well-designed self-report measures use wording and validation techniques specifically aimed at reducing these biases, rather than assuming self-report is unreliable by nature.