Sometimes you don't want to read a whole article to get the gist of a new technology or tool. You just want a person who's familiar with the subject to tell you what it is and why it's good in 1-2 sentences. Here are three of those kinds of descriptions for software-defined networking.
An SDN setup can give you massive, controllable virtual networks to move data securely and seamlessly within data-centers and into an environment where data and applications are available anywhere. However, for smaller companies, this isn't really on the radar.
If it works the way I think it does, it's especially useful for handling multiple datacenters. Your servers recover to a different building, county, or state without having to fiddle with VLAN settings or IP subnet schemes at either location to make it happen, that could be a big win.
It helps enable a VMware-style "software defined datacenter." Essentially, the network config lives in the virtual cluster so it can be moved around to different physical locations and have all the networking setup follow the VMs. This is only a partial description, but it gives you a useful analogy.