A stencil is the template you use to send mail to others. It will always stay the same, which means your formatting and text will be consistant every time. For the parts that you do want to change - a name, a price, a personal message - you can use placeholders.
To insert a placeholder into your Stencil press the Insert Placeholder button and choose a name and placeholder type.
Placeholder Types
Text Placeholder: Text placeholders allow you to insert a small amount of text into your stencil.
Multi-line Placeholder: Multi-line placeholders allow you to insert a large amount of text using an editor that lets you format your text in various ways.