Please give me a simple explanation in this.

Another person who doesn't understand the difference between front and rear sync.

True, but it's what occurs AFTER the flash fires that matters.  As EXPLAINED, with front sync, the flash fires as soon as the shutter opens.  Therefore, any movement by the subject AFTER the flash fires, but before the shutter closes, will cause the subject to become blurry due to motion blur, thereby causing the subject to appear out of focus.

That problem does not occur when using rear sync, because in rear sync mode the flash fires IMMEDIATELY before the shutter closes, thus NOT allowing any subsequent movement by the subject to be recorded.

ROTFLMAO!  Stop spreading such nonsense...please!  The entire purpose of rear curtain sync is to insure that the subject will NOT appear out of focus due to motion blur, ESPECIALLY when using slow shutter speeds to capture ambient light.

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The Five 'Ps' of Photography:
*Proper Planning Prevents Poor Performance!*