TSRVToolButton.Hint

Contains the text string that can appear when the user moves the mouse over the control.

property Hint: String;

Use the Hint property to provide a string of help text either as a help hint, or as help text on a particular location such as a status bar. A help hint is displayed only if ShowHint=True.

If the toolbar has a hint area (below or above the buttons), the main property explaining the button for users is Caption.