How is Solr ranking determined?

In your Solr settings (admin/islandora/search/islandora_solr/settings), look at the Query Defaults tab.

Under Query Fields, you'll see something like: 

dc.title^5 dc.subject^2 dc.description^2 dc.creator^2 dc.contributor^1 dc.type

This is the list of fields that Solr will look at by default in a simple search, and how strongly they rate a match within each field. If your search term is in the title field, it will be ranked more highly than if your term is just in the abstract (dc.description). The higher the number beside the field, the more importance a match within that field is given.

You can adjust your weightings here by altering the numbers, and even add or remove fields. Test your weightings in your virtual machine before setting them on your production site.

For more detials, see the Solr Reference Guide.