Route redistribution is when you take a route from one routing protocol and inject it or distribute it into another protocol.  By default, routers only advertise and share routes with other routers running the same protocol.  So if you have 2 routers and one runs OSPF and the other runs EIGRP and you want them to know about each others routes, by default, they won't share routing information because they are not running the same protocol.

So, you use route redistribution to 2 different protocols can still share and advertise routes to each other.