According to Opendefinition.org:
"A piece of data or content 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."
Open data is often associated with government datasets and APIs, but it can apply to any type of data created by anyone for any purpose. This data could be used to help answer research questions — such as "does improved state funding improve school performance?" It could also be used to drive 3rd party apps such as tracking apps for SEPTA that use SEPTA's API.
This guide provides a range of data resources along with considerations for when you are searching for or using data.