iter forward by a single cursor position.
Cursor positions are (unsurprisingly) positions where the cursor can appear. Perhaps surprisingly, there may not be a cursor position between all characters. The most common example for European languages would be a carriage return/newline sequence.
For some Unicode characters, the equivalent of say the letter “a” with an accent mark will be represented as two characters, first the letter then a “combining mark” that causes the accent to be rendered; so the cursor can’t go between those two characters.