The project usage overview page reports week-by-week usage of Drupal core and of contributed modules and themes. Usage figures are a count of sites using the item that week. The results shown give an idea of how popular the different projects are and may help you choose modules and themes for your own site.
As always with statistics of this sort, they don't tell the whole story. For example, the results shown for "Drupal" are a poor estimate of the number of Drupal sites on the internet. This is because only sites running the Update Status module report data back to drupal.org. Update Status has been included in the Drupal download since the 6.0 release (and the installer prompts the user to allow it to be enabled when the site is first installed). In Drupal 5.x it is only available as a contributed module. This means firstly, the actual number of sites running Drupal is higher than the figures would suggest, and secondly, the results are heavily biased towards sites running Drupal 6.x and later.
Even so, the usage figures reported are a really useful indicator of what other people are using on their sites, and may suggest some modules or themes that you should investigate.
At present, translations and theme engines are not monitored by Update Status and so their usage data is not recorded.
Note that the usage pages themselves are generated by the Project module. If you have suggestions or bug reports about the display of usage statistics, please search in the Usage statistics component of the Project module issue queue and if there is not an existing issue about your idea, please submit an issue there.