Differential Signals (LVDS)
What is the significance of common mode voltage in diff signals.


The common mode voltage is the average of the voltage of the two components of the differential signal. Ideally it doesn't vary when the signal is in a logical '1' or '0' state.

It is important to know the common mode voltage because different receivers might have different ranges of acceptable input common mode voltage and you want to know that your transmitter and receiver are compatible.


  Also, what are VoH &amp; voL meant for??


VOH and VOL are exactly what they are defined to be in your graph: the voltage levels that the signals switch between, depending whether the signal is a logical '1' or '0'. 

Again, you might need to know these levels to determine if your differential transmitter and receiver are compatible. For example, you wouldn't want VOH to exceed the maximum instantaneous input voltage of your receiver. 

The difference between VOH and VOL also tells you the amplitude of your signal. Sometimes you might need to check these values to know if the signal amplitude specified elsewhere is a differential or single-ended amplitude.


  where does the equal &amp; opposite voltage be specified in this figure.


If you subtract off the common-mode voltage from each component signal, then they will be equal and opposite. 

If the two components are \$V_A\$ and \$V_B\$, whenever \$V_A\$ is at \$V_{CM}+0.175\$ then \$V_B\$ is at \$V_{CM}-0.175\$ and vice versa.