What is the actual meaning of Zawwal? VS Istiwa?

Answer

Istiwaa means Zenith, i.e. the middle point between sunrise and sunset. For example if sunrise is 6 a.m and sunset is 6 p.m then noon is zenith.

Zawwaal is the starting point when the sun decline from zenith to move towards sunset.

To calculate it one may pitch an object on a flat surface. Normally the shadow of that object will then cast in the west direction and it will gradually diminish. The point where the shadow stop to diminish will be zenith / istiwaa, and the time when it starts to increase that is the time of Zawwaal.