Open data is, basically, the idea that certain data should be freely available to everyone to use as they wish, without restrictions from copyright, patents or other mechanisms of control. A piece of data is open if anyone is free to use, reuse, and redistribute it — subject only, at most, to the requirement to attribute and/or share-alike.
Sir Tim Berners-Lee, the inventor of the World Wide Web talks about open data in this TED talk:
“Opening up data is fundamentally about more efficient use of resources and improving service delivery for citizens. The effects of that are far reaching: innovation, transparency, accountability, better governance and economic growth.”
The idea would be: if you make your datasets open to the public, more researchers would have the opportunity to play with it and see what gives; potentially expanding knowledge. This can be particularly helpful for those operating on limited resources of their own: a lot of researchers using open data come from the global South/open data is being very successfully used by research informing policymakers in developed countries; see, for example, Ghana’s open data initiative or the Open Data Research network.
Now, Sir Berners-Lee makes another interesting distinction: