Can anyone tell me what the heck this is and if I should really worry ? Front passenger side wheel well

 

Need more information on your car to verify, like 4-cylincer/V6 or Automatic/manual transmission.

It looks like the transmission mount to the sub-frame. Is that crack in the rubber or metal?

 

V6 automatic
Crack is in the metal I believe.

This is going to sound like a foolish question but here goes: How high on the priority/emergency chart does this sit? Should replacing it be done ASAP?

 

That looks like the left side of the car, judging from the position of the axle. Are you somewhere with right-hand-drive? If so, some of this might be a little different depending on what country...

It's the engine mount (#10 in the picture). I would get it replaced soon because it can allow the engine to move around and push/pull on things like the exhaust pipes and the AC & PS hoses, wire harnesses, etc.

 

The car is drive-able, but I'd get it replaced in the near future. As JimBlake said, the engine could move more than normal and could cause some issues.