Body is generally described as the "mouthfeel".

In my experience, alcohol is the major contributor to body, but it is
not the only one. The viscosity of the wine (i.e. motor oil is more
viscous that water), glycerine content, and other factors contribute as
well.

I would say that if you are comparing 2 wines that are balanced, the
one with higher alcohol content will be the fuller-bodied one almost
every time. If the wines aren't balanced, then a higher alcohol wine
can "feel" thinner and less viscous than a lower alcohol one.

The best way to learn how to distinguish body is to drink a lot of wine
with someone who is very knowledgeable about the subject...