Stock Music goes by several different names, it’s know at Production Music, Royalty free Music and sometimes Library Music.
Stock music is music that is pre-recorded and kept “in stock”. It’s used for TV, radio, films, advertising, videos, presentations etc.
Lets say you wanted to have some music in the background of a TV advert, instead of going to the time and hassle of hiring someone to create the music for you, you could go and listen through some pre-recorded music in a stock music library with the style of music you had in mind to fit your content. When you’ve chosen the music you want, you pay a licence fee to allow you to legally use and broadcast the music that you’ve synchronised to your video.
For more info check out https://en.wikipedia.org/wiki/Production_music