When we say a body has "high" electric potential and "low" electric potential, to what potential is it with respect to?
When we use the term electrical potential we actually always mean a potential difference. The absolute value of the electrical potential has no physical meaning since we can never measure it. We can only measure potential differences. Technically the potential has a gauge symmetry, though this is possibly getting in deeper than your question warrants.
The electrical potential difference has a very simple physical meaning because it is just an energy that is the work done to move a unit charge. For example in your previous question you asked about the potential of an isolated sphere. To calulate this we simple take a test charge $q$ and calculate the work done to move this charge from infinity to the surface of the sphere. Then we divide this work by the charge $q$ and the result is the potential i.e. the work per unit charge.
Alternatively, and again going back to your previous question, the potential difference between the two plates of a capacitor is just the work done in moving our test charge $q$ from one plate to the other divided by $q$.