It's just a little thing that describes the sort of damage that Monks do in FFXI.
Basically, we hit very rapidly but for a lot of small damages. If you focus on each individual hit, you'll likely be underwhelmed by the damage we do.
However, if you instead add up the damage we do over time, and compare it to others job's damage over time, you find that we can be some of the biggest damage dealers in the game.
One of the favorite examples in the MNK world is a comparison of MNK to DRK (why DRK such animosity towards DRK? Well, a lot of DRKs pre-60 view their job as the biggest DD in the game, and they can be rather cocky with it. Thus, there's a lot of hatred for DRKs from many of the DD jobs. It's silly, and stupid, but there you have it).
MNK hits enemy for 25 points of damage.
MNK hits enemy for 50 points of damage (critical).
DRK hits enemy for 100 points of damage.
Person1: Woah!
Person2: {All Right!}
MNK hits enemy for 25 points of damage.
MNK hits enemy for 50 points of damage (critical).
MNK hits enemy for 25 points of damage.
DRK misses enemy.
MNK hits enemy for 50 points of damage (critical).
MNK hits enemy for 25 points of damage.
MNK hits enemy for 25 points of damage.
DRK hits enemy for 100 points of damage.
Person1: YAY!
Person2: Bah! Kick the MNK, he isn't doing any damage.
As you can see from this example, if you add up the damage the MNK is doing, he is outdamaging the DRK in the same period of time (275 to 200). But because everyone is always so focused on the individual hits, people don't see it.
Now, this is a problem most MNKs will face at one point in their career or another. Personally, I experienced it from approx. 40-60 or so. However, as I approached end-game, I noticed I experienced less and less of this. I would say that 65+ I never once experienced any DoT descrimination.