A V engine is a common configuration in larger engines in which the pistons are aligned so that if viewed from the front appear to be in a large V. The 12 refers to the fact that this configuration has 12 cylinders.
A number of World War II fighter planes used V12 engines, usually generating about 1,500 horsepower. Their use in fighters disappeared quickly after the advent of the jet engine.
There are also V6 engines, V8 engines, etc. The V configuration helps minimize vibration. V-twin (two cylinder V configuration) engines are common on motorcycles.