A witness is someone who watches you sign your signature on your own Will.
Many people are unsure who can witness their Will. Using the Australian Will Kit, any person who is over the age of 18 can witness you sign your Will. The witness cannot be a beneficiary of your Will, nor the partner of a beneficiary (see more detailed information in the Australian Will Kit).
The Australian Will Kit has a complete section on how to sign your Will and who can be a witness. It is well worth reading all the rules before you write your Will.