マルチCPUで平行処理をするもの。マニュアルでスレッド数を指定する事もできますが、threads=auto でCPU数に応じて分散数を自動的に決めさせる方が良いでしょう。マルチプロセッサではリニアに速度が向上する（コア一個に付き約94%）ので絶対使った方が良いです。画質劣化は僅かなものです（デュアルコアで約 0.005dB 、クアッドコアなら約 0.01dB ）。
This option allows to spawn threads to encode in parallel on multiple CPUs. You can manually select the number of threads to be created or, better, set threads=auto and let x264 detect how many CPUs are available and pick an appropriate number of threads. If you have a multi-processor machine, you should really consider using it as it can to increase encoding speed linearly with the number of CPU cores (about 94% per CPU core), with very little quality reduction (about 0.005dB for dual processor, about 0.01dB for a quad processor machine).
