x264 is an open source encoder for the H.264 video compression standard.
x264 is written in C and is a software based video encoder. It is a CPU focused so it doesn't utilize GPUs for processing videos. The source code for x264 lives at https://code.videolan.org/videolan/x264
There are some competing implementations for H264 that use the GPU for processing videos:
Generally, x264 is the right tool if you want higher quality H.264 encoding. If you want fast encoding at the expense of quality, you should opt for a GPU based encoder.