‘Position’ column in product csv

Hi
Forgive me if this is a silly question – but what does the ‘Position’ column in the product import/export csv refer to? When I originally imported products it was set to zero; I’ve since added more, & when exporting them all I’ve noticed that some of them now have consecutive values from zero up to over 1000, while some are still zero – yet I can’t see any difference in product layout etc. Looking at the sample csv at Github, again some are zero, some not.

Thanks
Pete