If you don't play games you might not be familiar with the term. A "tank" is a guy that has heavy armor, attracts the attention of the enemies and is meant to absorb a great deal of the damage as they can take more of it. The healer is meant to focus moreso on them and this is their major if not entire job. The other members of the group are DPS (damage per second) and a healer.

source
Last night I found myself in a situation where the tank in our group had no idea what he/she was doing and it was a disaster. You see, as an archer, i am meant to stand back and deal out damage and handle any "adds" (additional enemies that enter the fray.) I am not meant to be taking massive hits from the bosses. I simply can not endure them.

source
The "tank" has certain abilities that can quickly make the enemies focus back on attacking him / her should the enemies decide to focus on the other members of the group and I can only assume that this guy was not using them because i was getting annihilated. At several points I was taking most of the damage and the healer couldn't heal me fast enough and I died (the healer was doing their job 100% correctly, but the spells have timers, you can't just use them at will.)

source
The boss was at the end of the second dungeon called The Tam-Tara Deepcroft. It is normally very easy as it is meant for low-level characters. It was chaos as the tank was not doing their job at all. At times, the mobs were all over the healer and this is something that you absolutely must take care of immediately because well, if that guy dies, you are done. I had to focus on these opponents to make them focus on me instead. This is what the tank should have been doing as I am only marginally better at taking damage than a White Mage is.
I was trying to offer advice to the tank but I don't think that he/she was even reading the dialogue because his/her tactics did not change at all. Perhaps it was a little kid or someone who doesn't speak English. I dunno. We ended up finishing the dungeon in the end but not until i had been killed 3 or 4 times (In an average group no one will die at all.)
This was a bit frustrating to me because FF14 is NOT a hard game. The tutorials are excessively long and I simply cannot understand how a tank could play the hours and hours required to gain access to a dungeon without learning the basics.
I suppose the underlying message of this article is that if you decide to play this game (and you should, they recently dropped the price AGAIN, which might be why we are seeing so many new, novice player) and you decide to be a tank. Please please please learn your role in the group.