Most of the video you’ll come across is compressed, meaning its been altered to take up less space on your computer. For example, a regular Blu-Ray disc usually takes up around 30 or 50GB of space—which is a lot for a normal person to download or store on their hard drive. So, we compress movies to make them more manageable, usually with some loss in video quality.
A codec compresses and decompresses data. It interprets the video file and determines how to play it on your screen. Your computer comes with many codecs pre-installed, though you can install codec packs for wider support, or a program like VLC or PotPlayer (which we prefer to codec packs). Some examples include: [read]