Your question is not clear about what you are asking for, but I will try to guess what you mean.
You want a read-only line of text in JIRA - if you really do mean a field with variable content, then you have two options.
In both cases you are going to have to think about how to get data into them, and you must remember that they will not appear on edit screens, only view (and searches and reporting etc)
The other thing you might be looking for is some form of static text on create or edit screens. To do this, install the JIRA Toolkit add-on and use a "message field"
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