What is the purpose of a tri-state pin in a Oscillator
There is a line in the Parameters table on p.1 of the datasheet, which describes the function of the Tri-state.



It's a kind of an enable-disable pin.  


If Tri-state pin is logic "1", then oscillator is connected to the output pin.  Same happens if Tri-state is left unconnected (there's probably an internal pull-up).  
If Tri-state pin is logic "0", then oscillator is not connected to the output pin, and output is floating.  The fact that it's floating may be useful if you need to switch between several clock sources.  You would enable one of them, and Tri-state the others to prevent contention between multiple outputs.


edit:
Here's an example where an enable/disable with tri-state is used with an external clock source.  When connecting external clock source to J1, the J2 jumper should be installed.  The resistor R1 protects from a direct short between clock outputs.


(schematic from p.22 in this user guide and datasheet for that oscillator)