When a video is uploaded or prepared for download, the codec compresses the raw data to make the file smaller.
Without these tools, a single hour of 4K video would take up roughly of space—far too large for any standard hard drive or internet connection. Popular Video Codecs for Downloading video download codec
When you hit "play" on your device, the codec decompresses that data so you can actually watch it. When a video is uploaded or prepared for
Choosing a Video Codec for Optimal Quality and File Size - DivX and device compatibility.
Choosing the right codec is a balance between file size, visual quality, and device compatibility.