\addcontentsline command adds an entry to the specified list
or table where...
fileis the extension of the file on which information is to be written:
toc(table of contents),
lof(list of figures), or
lot(list of tables).
sec_unitcontrols the formatting of the entry. It should be one of the following, depending upon the value of the file argument:
toc- the name of the sectional unit, such as part or subsection.
entryis the text of the entry.