open source project

Hi Guys,
        
        My dream is to do Masters in a good college in US.But when i was checking their prerequisites for the admiision, they are asking for strong knowledge in C/C++,Datastructures,Algorithms,Contribution to OPEN SOURCE PROJECT. what they exactly mean by OPEN SOURCE PROJECT. i am CSE GRAD with a GPA of 6.5. All i have got is 2 years of work experience in a service based company and  i was working on .NET platform.
please help me on how i can start working on OPEN SOURCE PROJECT.



Thanks & Regards,
Sivapratap