On its own, the Text command displays a simple dialog box with a line of text (left-aligned) inside. It can also be used inside a Dialog..EndDlog block, to add a line of text to a more advanced dialog.
Text will give an error if the string is too long — how long varies from model to model, and depending on if Text is being used inside or outside Dialog..EndDlog, but in general anything below 30 characters is safe (otherwise, you should test the dialog first to make sure everything fits). It uses the small, variable-width font on the TI-89 and TI-89 Titanium, and the normal fixed-width font on widescreen calculators.
You can add Text "" to a dialog to skip a line between two other elements.
230 - Dimension happens when the line of text is too long to fit in a dialog box.