When soldiers debouch, they march out of a very narrow space. Troops might debouch out of a long pass between mountains, for example.
When military troops debouch, they march in a long, narrow line — sometimes single file — because of the width of the area they're moving through. As they debouch, the soldiers emerge in a larger area, like an open field. The narrow space is known as a defile in military terminology. The verb debouch has a French root, déboucher, which combines dé, or "removal," and bouche, "mouth."