ffmpeg -i input.mp4 -c:v libx264 -preset medium -crf 23 -c:a aac output.mp4

Re-encodes the video with the libx264 codec (algorithm that compresses video) and audio with AAC. The -crf 23 flag sets quality: lower numbers give better quality and bigger files, 18-28 is the usual range. -preset medium trades encoding speed against file size, since slower presets compress better at the same CRF. A solid all-purpose conversion recipe.