You may wish to begin by reading the text widget conceptual overview which gives an overview of all the objects and data types related to the text widget and how they work together.
A TextMark is like a bookmark in a text buffer; it preserves a position in the text. You can convert the mark to an iterator using get_iter_at_mark. Unlike iterators, marks remain valid across buffer mutations, because their behavior is defined when text is inserted or deleted. When text containing a mark is deleted, the mark remains in the position originally occupied by the deleted text. When text is inserted at a mark, a mark with “left gravity” will be moved to the beginning of the newly-inserted text, and a mark with “right gravity” will be moved to the end.
Note that “left” and “right” here refer to logical direction (left is the toward the start of the buffer); in some languages such as Hebrew the logically-leftmost text is not actually on the left when displayed.
Marks are reference counted, but the reference count only controls the validity of the memory; marks can be deleted from the buffer at any time with delete_mark. Once deleted from the buffer, a mark is essentially useless.
Marks optionally have names; these can be convenient to avoid passing the TextMark object around.
Marks are typically created using the create_mark function.