A ‘music bed’ refers to part of a jingle, or a whole version of a jingle itself, where sung vocals are not present. For example, in the case of the top and tail jingle, it refers to the middle section, where only instruments are heard.
Music beds allow a voice over to be spoken over the top, without the spoken words being interfered with by the sung vocals. This is why it is important to have a few different cuts of your jingle, so you can use each of the different versions for different purposes, depending on the context.
For more definitions, read the post jingle lingo, what does it all mean.