What is it
NVDEC is an API for Nvidia graphics adapters that supports decoding of a variety of video formats. It does not yet support GPU to Video direct rendering. It works for video up to 4K but not 4K at 50fps. It also includes support for GPU based deinterlace.
MythTV V31 includes a playback profile that supports NVDEC. In order to use NVDEC you need a supported Nvidia card and an up to date driver.
For a list of supported cards see https://developer.nvidia.com/video-encode-decode-gpu-support-matrix#Decoder.
You need to install driver version 396 or later. For ubuntu these can be obtained from the ppa:
sudo add-apt-repository ppa:graphics-drivers/ppa
After adding the ppa, use the "additional drivers" menu to select the latest driver, v 396 or later.