Over the past two practices, Morikawa sensei has been telling me that I need more seme. All I was doing when I go up with him is do various attacks, but not really doing much to gain control of the center before I go. This concept has been one of those harder things to grasp, so I thought I'd take a moment and do some research on it.
A quick look at the Japanese-English Kendo dictionary provided by Kendo USA says that seme is a noun meaning, "the retention of superiority in relation to an enemy through kiryoku, the shinai and datotsu." It's a pretty simple definition on the surface by basically saying that you are doing whatever you can to gain the advantage to make a successful attack. Personally, that doesn't really help me too much, so I decided to dig a little further.
The next source I looked at was this book I bought a while ago, which can be translated to The Kendo Improvement Book (剣道上達Book ISBN4-415-01915-3). It just happened to have a whole section devoted to answering my question so I decided to take a look. I would like to note that this book is in Japanese, so I apologize in any errors in translation ahead of time.
In this book, seme is part 1 in a series of things that needs to be done before you attack--Seme -> Tame -> Search for opening (Kuzure)-> Attack. The seme here has been defined mostly as the process of taking the center by making sure the tip of the shinai is pointed toward your opponent's midline. They also made note of the fact that seme isn't just the act of getting the center. In addition to that, you also need to pay attention to your opponent because they will be trying to get the center as well. Then you still have to pay attention to any moments where openings occur on your opponent as well as being aware of yourself in the process. There are also pictures in the book that show some common things that your opponent might do that will open up several things.
This is one of those concepts that has a short definition to get the basic point across, but there are some deeper things that one needs to consider in order to use this effectively. The above definition is by no means an exhaustive description of what seme is, nor do I consider myself an expert in what it is. But at least it is a start for me to actually begin to effectively use this in practice to gain and maintain the advantage before and during a strike