When a video camera is recording, it "sees" each video frame as a grid-arrangement of separate pixels, each with its own brightness and color values. Each frame of pixel values is stored in a video file on the camera's media - usually a memory card, but sometimes an internal hard drive, DVD disk or magnetic tape.
Camera media is still fairly expensive so we need the memory card to hold a reasonable amount of recording time. However, all of this individual pixel data takes up a lot of storage space, so the memory card would fill up quite quickly if this "uncompressed" pixel data was just dumped directly into a video file.
This is why nearly all video cameras contain a video encoder that compresses the pixel data before recording it, and a corresponding video decoder that decompresses it again so that the original pixel data is recovered and the frames can then be displayed on the camera's screen. So, the video encoder is used during recording and the video decoder is used during playback. The encoder uses a particular compression scheme to reduce the size of the original individual pixel data. "Compression scheme" really just means a method of compressing the pixel data - there are lots of possible methods that could be used but a particular encoder will usually use just a single compression scheme.
Obviously the decoder in the camera must use the same compression scheme as the encoder, so that the decoder can correctly reconstitute the original pixel data. Because encoders and decoders work as a matched pair, the pair are often referred to collectively as a "codec" (that is, enCOder/DECoder).
The compressed video files can be copied to your computer, but they can not be played back directly because the video is stored in a compressed state which is unintelligible without the availability of a suitable matched video decoder. So, to play compressed video files on your computer, the computer must have a video decoder that uses the same compression scheme as the camera's video encoder.
Many video cameras also compress the audio stream in a similar way using a separate audio encoder, and the computer must then have an audio decoder that uses the same compression scheme as the camera's audio encoder.
Video decoders and audio decoders that can be installed on a computer are available from many different third-party suppliers - some are paid-for and others are free. Also some versions of Microsoft Windows come ready supplied with some decoders.
There are also some decoders built into the Intel processor (CPU) that is at the heart of most computers and laptops that were manufactured in the past few years.
DVMP Pro has a set of options that can be used to select which of the several decoders that may be present on your computer are to be used when playing or burning-in files that use the corresponding video and audio compression schemes. For the sake of convenience, most of these are selected automatically when DVMP Pro is installed, but you can change these later on if you need to.