A last will and testament is an important step in planning for the distribution of your estate (assets, both real and personal) upon your death. A will and last testament is a written legal document by which a person, the testator (you), expresses their wishes as to how their property is to be distributed at death, and names one or more persons as the Executor to manage the estate.
 
The Will is submitted (lodged with) the Court and your Executor files with the Court to begin the probate process. The Will is the "instructions" to the Court as to how you wanted your estate (your stuff) to be distributed (given) to those named in the Will.
 
Your heirs will have to go through the probate process with the court in order to receive any distribution of your estate (if not a designated beneficiary).  A Will is not required, but without one, the laws of intestate succession will determine the distribution of your assets. The distribution through intestate succession may not be to the people you desire. Thus, a Will sets forth your wishes and distribution of your assets accordingly.