It can mean different things to different people.

If you're a distributed systems developer, you can use Mininet to create a topology and emulate the interaction of 100s of, say, bittorrent clients.

If you're a graduate student in networking, you can use Mininet to experiment with your new routing protocol and compare it against existing ones such as OSPF, BGP, etc.

You can also use it to put existing research papers to the test: http://reproducingnetworkresearch.wordpress.com