Creates a new line and inserts it at the beginning of the store. The first parameter parent can be null. If that's the case, the new entry will have no parent and thus be a "root" entry.
The second parameter items is an array with a value for each column. If your treestore has three columns, the array should have three values.
The method returns a GtkTreeIter which you can use to refer to the entry row; e.g. to use it as a parent for another row.
If you pass no parameters at all, an empty row with no values will be created, and you have to use other methods like set() to change the column values.