This is a basic distance in kendo and also ultimate distance in kendo. It is literally means one step and one cut.
If you take a step forwards, you will be able to cut your opponent with one cut.
Thus, in issoku itto no ma is a distance of one step and one cut. "ma" means a distance in this case.
When you face a person while both of you have a shinai, the end of your shinai and that of your opponent are touching.
The point of both kensaki touching is also known as the point of "dead or alive."
Once you take a bit forwards beyond that point, either you or your opponent is dead or alive.
Thus, issoku itto no ma should be taken into account more seriously. A lot of people do not do anything beyond that point.
I used to get told off because I did not strike after passing the point. But of course, I had my reason, which was "I did not see any opening."
However, Tsurumaru sensei told me, "It does not matter if you win or lose once you pass that point, because winning or losing should have been decided before you pass that point."
Do you get it? I could not for a while since I was 12 or 13 years old back then. However, now I kind of know. I think what he meant was that "you should win before you strike."
How? You may ask. It is by taking the center of your opponent, causing one of the shikai states of the mind in your opponent's mind, or by putting a lot of pressure onto your opponent through your shinai.
It is hard to do, of course. However, imagine how perfect it would be? You do not have to strike but you defeat your opponent. Man, it is deep.