This indicates High Definition (HD) resolution (1280x720 pixels), providing a clearer picture than standard broadcast .
"ReqZone" is likely the release group that encoded the file, and .mkv (Matroska) is a flexible container format that often includes multiple audio tracks or subtitle options. Where to Watch Officially
This refers to the video compression codec (H.264), used to maintain high visual quality while keeping the file size manageable.