Framework provides you set of rules for implementing your application and also provide some of the code to support that.
STRUTS is a framework built on MVC
pattern and it has some rules/requirements and also provide bases classes like ActionForm and Action etc
Open Source means something that is freely available and its code is also available for you and anyone can change in it for his requirements and can also share that for others having the same problems. Struts is open source and its code is available in CVS.