Here's a standard definition for this French military term -
To appreciate the great advantage of enfilade fire, let's consider the standard situation: two opposing firing lines which are nearly parallel with one another. In this case, shots fired from one line towards another may fall short, pass by or pass over intended targets. Chances of a miss are fairly high. We may increase chances of a hit by shortening the range between the two lines, and maintaining a high volume of fire to compensate for the misses.
Enfilade means that one firing line forms an angle with another, all the way to being perpendicular. A rifleman firing down a line might miss his target by overshooting it, but still have an excellent change to hit another target along the same line. A shot fired towards a target in the middle of the line could fall short and still hit another target. A shot fired from enfilade will be unlikely to pass through a line without hitting something.
Whether in line or column, to avoid the danger soldiers attacked from enfilade (or on one flank) will scatter or retreat in disorder. They are quickly and easily defeated, usually with small loss to the attackers. Officers write about "rolling up" an enemy line, exactly what appears to happen.
Examples are Jackson's attack at Chancellorsville VA (May 2, 1863), which also enjoyed the great advantage of surprise, and Crook's attack on Early's line at Fisher's Hill VA (Sept. 22, 1864).
I hope this helps!