This is a terminology that you cannot live without as long as you do kendo. Let me tell you the meanings for each word.
Ki: In English, a Chinese word, chi, is more well-known than ki.
We have the same kanji for it. However, it could be a bit different.
The Japanese have two ways of writing it.
However, they both mean the same.
Ken: sword, including shinai, bokuto, shinken and etc.
Then what does "ki ken tai icchi" mean?
It means that "to synchronize the movements of ki, ken, and tai".
It is hard to see the ki movement. So ki usually refers to your shout.
When you strike men and you do not say "men" then you are a lack of ki. You must declare where you are striking. In that sense, you cannot strike kote and shout "men!".
You have to declare where you intend to strike and synchronize ki with your shinai and body.
Once you put everything together when you strike, your strike is considered to be valid.
It does not matter how many times you actually "hit" men, kote, do or tsuki, you cannot get a point unless you have them all together.
Without the word ki, so ken, tai icchi, it means different so please be aware of that.
If you want to know the meaning of "ken tai icchi" click here. And also some may say "ki ken tai no icchi". "No" here means "of". So it means "synchronization of ki ken and tai".