What an audit is
A smart contract audit is a manual and automated review of a project's code by security researchers, looking for bugs that could lose or lock funds. A good one produces a report listing findings by severity, what was fixed, and what the team chose to accept. The report is the product, not the logo.
What it does not guarantee
An audit covers the specific code, at a specific commit, at a specific time. Ship new code after the audit and that code is unaudited. It also says nothing about whether the team will rug you, whether the tokenomics are predatory, or whether the admin keys are controlled by one person on a laptop. Plenty of audited projects have been drained, usually through something outside the audited scope.
How to read one
Check the date and the commit hash against what is live now. Check the firm. Read the high and critical findings and whether they were fixed or just acknowledged. A report full of accepted criticals is a tell. One clean audit from a serious firm beats three vague ones from shops nobody has heard of.