Section 57 of the Migration Act 1958 is a procedural fairness obligation. Before the Department of Home Affairs refuses a visa application on the basis of information that is adverse to the applicant, it is legally required to first give the applicant an opportunity to respond to that information.
In plain terms: the Department has found something it doesn't like about your application. The Section 57 letter is its formal way of telling you what that is before making a decision. It is not a refusal. It is an invitation, and it may be one of the most important documents you ever respond to.
The letter will set out the specific adverse information the Department is considering. You then have a set time, usually 28 days, to provide a written response and any supporting evidence. The Department will take your response into account before making its final decision.
If you do not respond, or if your response does not adequately address the concerns raised, the Department will typically proceed to refuse the visa based on the adverse information it holds. A refusal can then be appealed to the Administrative Review Tribunal, but it is always better to prevent a refusal at this stage than to challenge it afterwards.
The quality of your response to a Section 57 letter is often the single greatest determinant of whether your visa is granted or refused. This is not a step to handle without professional guidance.