Open data is raw information that is readable by machines and made public with a license.
Let’s have an example:
RAW INFORMATION: The World bank publishes The Changing Wealth of nations -report as open data. They have estimated how much wealth each nation in the world has. So, this is raw information.
PUBLIC AND STRUCTURED: You can download the data as excel spreadsheet file here. So they have published the data in the internet and it can be processed in spreadsheet program.
LICENSE: And they have a licence here. It says you can do almost anything with the data.