What is meant by relative voltage?
Voltage is always relative. When you just hold one probe of your DMM to the + of a battery, for instance it will display 0 V, like it's saying "this is nothing, what do I have to compare it with?".  

Any circuit should have a ground (yours hasn't). Ground is your reference everything else is compared to. Usually that's the negative terminal of the power supply. It's only by calling ground 0 V that the supply's +12 V makes sense: it's 12 V higher than ground. If you measure 2.7 V in your circuit the "referenced to ground" is implied, unless indicated otherwise. That's the case when you say "relative to X", then you don't use ground as a reference, but the voltage at X. 

For DC normal arithmetic applies: if A is +5 V relative to ground and B = +12 V relative to ground then A is -7 V relative to B.