x264: This refers to the video encoding used. x264 is an open-source encoding tool that provides high-quality video at lower file sizes. It's widely used for distributing H.264/AVC encoded video.
When you see a string like this, it identifies the quality and format of the video: Hereditary (2018): The title and release year. hereditary2018720phinengblurayx264esubsmkv
x264: The compression codec used to encode the video, making it playable on almost any modern device. File Specification Breakdown