Data that is either openly-licensed, or in the public domain, and stored in online repositories where the datasets are free to access and download.
Open, freely-usable data makes activities such as 'Open Science' much easier to carry out. For example, one researcher's data could become useful to a different researcher years later, when analyzed in a different way, or perhaps in aggregate with other studies' data.
Resources that will help you learn more about and work with open data:
Licenses for data have been found in various places:
Open Access Directory: Data Repositories
Registry of Research Data Repositories (re3data)
Open Knowledge International Projects
Nature: Recommended Data Repositories
NIH: Data Sharing Repositories
NIH-supported data repositories for specific biomedical data types
PLOS: Recommended Repositories