When you create e pre-designed template you can apply restrictions on what your customers can edit on the design. You can restrict editing options on the whole template and/or on each element of the design.
One of the needs you may have is to set only text on your product with the ability to customize it.
How to do
Go on Tools > Pre-designed templates click on the product > click on.
Add your text on your product, click on and you can set the options of this element. (see below)