Did you place this text on the master page? If so, it will be displayed on every document page that is based on that master page, and will be locked. If you want to edit the text within the frame on document pages, you need to release those frames. You can do this on a frame-by-frame basis by clicking on the frame with the Selection tool while holding Shift and Command (if you're on a Mac, but maybe Shift and Alt or something else if you use Windows—just try a few combinations and you'll find it). If you want to release all master page items, go to the fly-out of the Pages window and select Override All Master Page Items, and it will release items on which ever pages you have selected in the pages window, so make your selection of pages appropriately.
If you want to keep a link to the frame itself, don't move or change the shape of the frame on the master page. If you retain the link, you will be able to change the position or shape of the frame on the master page, and all unedited frames on document pages will move or change shape accordingly. Any edited or moved frames will act as if you created them on the document page.
There are various opinions on how to best use master pages. Some like to only place items that won't change on document pages, and others find it helpful in some situations. It might help you to search out some of these opinions on the forum to see what works best for you.
P.S. Other reasons you can't edit something is that the object might be locked, or on a locked layer, or both. Just FYI on that, but my guess is the master page answer.
Michael Gianino wrote:
maybe Shift and Alt or something else if you use Windows
it's Ctrl+Shift on PC