When you steal a march on someone, you do something before the other person, and as a result you gain the advantage. Very often, your competitor isn’t even aware of what you are up to. You accomplish your task secretly, without being noticed.
We stole a march on our rivals by launching the product a month before they did. The expression comes from the world of medieval warfare. I understand that the word “march” refers to the distance that an army could cover in a single day.
The word “steal” means to do something without the other person being aware of it. In the old days, in order to catch their enemy off guard, generals used to march their army during the night. By doing this, they very often succeeded in surprising their enemy. They literally “stole a march” on them.