In the Slavic languages and some other languages including Old English and Sanskrit, the instrumental case has the basic meaning of 'being used as an instrument (means, tool) to achieve/accomplish something (an action specified by a verb)'. The instrument can be some physical object, but can also refer to an abstract concept.
Sources
- Instrumental Case in Tamil language
- The Instrumental Case in Russian language
- What is instrumental case?: http://www.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsInstrumentalCase.htm
- Dictionary.com: http://dictionary.reference.com/search?q=instrumental%20case
- Prepositions Governing the Instrumental Case in Russian language: http://www.departments.bucknell.edu/russian/language/prepins.html