Yeah as itimpi says they are the number of the book in the series.
In Calbire on the metadata window where it says Series: you should enter the series name, for instance I have all the starfist books so my series is called Starfist Series.
Then under this there should be a box saying Book 1.00, this is the number in the brackets and means it is the 1st book in the Starfist series. Use the arrows next to it to increase this number.
So for example if I was editing the metadata for Book 4 in the Starfist Series it would look like this:
Series: Starfist Series
I also put the book number in the Title of the book for easy referencing.