There are a few options depending on your need.
Each field has a field help text box that you can enter text into for display when a user hovers over the "i" icon:


You can also display a message when a user opens a form using a dynamic form rule:


You can also use the image on load (IOL) technique to do a variety of things. This post has the details; be aware that Quick Base does not support this capability and it could lose functionality in the future:
https://community.quickbase.com/quickbase/topics/how-to-dialog