A summons is a legal document telling you that you are required to attend a particular court at a particular date and time. Courts use several different terms to refer to the document. If you are the defendant in a criminal case, it is called a court summons. If you are the defendant in a civil case (meaning another person or organisation is taking action against you), it is called a claim form. If you are a witness, the document is called a witness summons, though it's also referred to as a subpoena. A summons usually comes through ordinary post, so it is possible you may not receive it at all, or may not receive it until after the relevant court date.
In most cases, failing to answer a summons means your case can be treated as if you appeared in court but did not say anything to argue your point of view. In a criminal case this could mean being found guilty in your absence and possibly given a heavy fine. For more serious offences, the case may be put on hold and an warrant issued for your arrest. In civil cases, the court may make a default judgment. That means the other side in the case is treated as having won the case, in the same way as if you had appeared and argued your case, but the court found against you.
If you did not receive a summons, you can make a statutory declaration. This is a written statement that meets the requirements of the Statutory Declarations Act 1835 and explains that you did not receive a summons. You will need to contact the court to arrange to make the statement. To make the declaration legal, you must read it out at a court in front of a Justice of the Peace; you both then sign the document. You must make the declaration within 21 days of the point at which you became aware that a summons was issued and you didn't receive it, for example when you get notice of the case judgment.
When you make a statutory declaration, the court will set aside ("void") the relevant proceedings. For example, if you were found guilty in a criminal trial, the case is treated as if the hearing didn't happen. This doesn't mean the case itself is closed: the prosecutors or the people bringing a civil claim have the right to restart the legal proceedings and you may still face a hearing.
Do not try to use this procedure to pretend you didn't receive a summons if you actually did. The point of a statutory declaration is that you are making the statement legally in front of a court. If it later emerges that you were lying when you made the statutory declaration, you face prosecution for perjury. In cases involving a statutory declaration, the maximum sentence for perjury is two years in prison.