The 'String' property contains the string (character vector), which is displayed in the GUI. The 'Value' contains a numerical value, e.g. 1 or 0 for a toggle button or check box to determine, if the control is enabled or disabled currently. For a slider it is the current value between the min and max values. For a listbox, 'Value' contains the selected rows.
While the 'String' is usually fixed, the 'Value' depends on the user actions (with exceptions, of course, e.g. if the name of a user selected file is displayed in a button).