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In simple terms, this option controls the maximum number of frames the CPU prepares in advance of being rendered by the GPU. The benefit of having frame data buffered in advance of being processed by the GPU is that it helps keep the GPU consistently fed with data, smoothing out any small variations in rendering times. The disadvantage of buffering frame data in advance of displaying it is that it can increase latency, i.e. a delay between your input via keyboard or mouse, and the resulting impact on the image displayed, commonly referred to as input lag.
também:
the impact of altering this setting appears to be quite complex. In some cases, lowering this setting may actually reduce stuttering or improve performance, likely on systems where the CPU is much slower than the GPU. So some experimentation is required to determine the optimal setting for each game on your system.