A string is simply a sequence of characters.   For example, the double quotes below enclose strings :

i.e. anything you can type on the keyboard.

Nodes have associated patterns.   A node's pattern can be a string.

The user can set case-sensitivity for a string with a checkbox.   If the "Ignore case" checkbox is checked case is ignored when the program attempts to match the string.   If the box is not checked case is considered when matching the string.