Indeed, 1st arg is index where to insert the text, 2nd is text, 3rd is tag, 4th is text 5th is tag - often used for text with non-trivial formatting...

here is an excerpt from tk docs:

pathName insert index chars ?tagList chars tagList ...? 
Inserts all of the chars arguments just before the character at index.
+ If index refers to the end of the text (the character after the last
+ newline) then the new text is inserted just before the last newline 
+instead. If there is a single chars argument and no tagList, then the
+ new text will receive any tags that are present on both the characte
+r before and the character after the insertion point; if a tag is pre
+sent on only one of these characters then it will not be applied to t
+he new text. If tagList is specified then it consists of a list of ta
+g names; the new characters will receive all of the tags in this list
+ and no others, regardless of the tags present around the insertion p
+oint. If multiple chars-tagList argument pairs are present, they prod
+uce the same effect as if a separate pathName insert widget command h
+ad been issued for each pair, in order. The last tagList argument may
+ be omitted.
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