Hello Upendra,

Google Summer of Code, abbreviated as GSoC is an annual international program that is held by Google for university students where they have to complete a free and open source project given by one of the participating organizations. The selections are done on the basis of the involvement and contributions to various projects by the students in the communities of various companies. It is conucted from May - August every year.

The only requirements are that you should be a university student and above 18 years of age. 

Some other open source programs include Outreachy, Ninja Winter of Code by IIT Kharagpur, KDE Summer of Code, Mozilla Winter of Security,etc. In addition to these you can always browse through Github and contribute to the various projects. 

Contributing to open source is a good step towards gaining professional skills as you would be working with others in solving issues and would also learn to write clean code that would be readable by others.