In what context ?
A textbox is, by default, an editable area of an online form.
If you want to have a textbox that may not be typed in your can simply add the following tag into the textbox snippet:
The whole snippet would look something like this:
<input name="" type="text" readonly="true" />
I'm not sure why you'd want a textbox that may not be edited unless you are going to populate it with dynamic content where you are using it to display a value pulled from another source.
Anyway, it's not for me to know why you want to do it. The above is how you can do it though. Hope it helps.