The ability to adjust the size of text displayed on the screen.
Larger text is easier to read from a distance, at a glance, and without glasses (for those that wear them.) Smaller text may allow more text fit on the screen at one time (although this depends on the phone and is not always the case.)
Not all phones have an option to adjust text size. Of those that do, many only allow it to be changed for certain functions. For example, a phone might allow you to set the text to be larger on the dialing screen, in messaging, and in the browser, but not for caller-ID, the contact list, or other menus. Only some phones allow all or most of the text to be made larger.