After confirming roles, you will be taken to a series of screens like this:
For each monster in a fight, you will be asked to assign a Tank, or Tanks. The 'Tank' is the person who will be attempting to attract the attention of this monster, so that it attacks them instead of weaker raid members. Warriors typically make the best tanks.
You can assign multiple Tanks to a single monster. This is usually done so that in case one of them dies, there is still a back up person to keep the monster from killing the other Raiders. Only Warriors can effectively Tank more than one monster at a time.
Each monster must have one Primary Tank, noted by the P in the symbol and on the display to the right. In the case of identically statted Tanks assigned to a monster, the Primary Tank will be the one to win out (if you consider getting beat on by a monster winning).
You can click on a raider to switch their Tanking status On of Off. Off being noted by the crossed out shield. Anyone set to Off, will not attempt to attract the attention of this monster. Alternatively, you can highlight a Tank with the up and down arrow keys, and then press the space bar to toggle Tanking status. The Primary button at the bottom of the screen, or the P key, will switch the Primary Tank to the currently highlighted raider. When you are happy with the layout, press the Confirm button(or Enter key) to continue on. Finally, the Back button will return you to the Raider Role Selection screen, instead of the Town Menu!