This is a concept which is in opposition to a “restricted war” or “limited conflict.” In that scenario, two countries are at war in a limited area, specifically seeking military out military targets, and trying to achieve a limited objective.
A “total war” is when two countries go to war with each other for complete domination, and make no differentiation between civilian and military people or targets. All objects and personell under the control of or aligned with the enemy are considered combatants and are considered legitimate targets.
In modern times, these are conflicts like the two World Wars, the American Civil War, etc.
For example, in 1864, General Sherman wrote:
[We are] not only fighting hostile armies, but a hostile people, and must make old and young, rich and poor, feel the hard hand of war, as well as their organized armies…
It’s commensurate with phrases like “scorched Earth” and “take no prisoners.”