A .prel is not a video file. It is a Premiere Elements project file -- which is made up of video, audio, etc.
You can output a video from a Premiere Elements project. How you do it depends on which version of the program you're using and what you plan to do with the video once you output it (ie, post it to a social media site, use it in another project, load it onto a smartphone, burn it to a DVD or BluRay).
In version 15, output options are found under the Export & Share button. Part 8 of my free 8 part Basic Training tutorial series shows you how to output videos.