What is testamentary intent?
One of the four general requirements for a will to be legally valid is that the person preparing the will must have testamentary intent, which means that they intend for the document to be their will. For example, if you write a letter to your brother asking him to prepare a will for you, that letter would not meet this requirement because you did not intend the letter itself to be a will, you just intended it to be instructions for what you wanted in your will.
The other three general requirements for a will to be legally valid are that the person preparing the will must have legal capacity and testamentary capacity, and the will must be prepared pursuant to specific formalities.