Yep, read the article above.
You can only assume that it means they have a flat frequencey response, which mean thst no particular frequencies are emphasised or cut. They are specifically meant to provide you with a representation as true as possible, without any colour (ie. distortion) provided by the speakers.
You will generally find that you can hear more detail with a decent pair of monitor speakers/headphones. If music is poorly mixed or produced, it will show up theses flaws.
If you produce and mix, then it is important. If you are just a music listener, then not so much - unless you value sourcing quality music on a decent format and hearing it as intended (ie. not on mp3).
For example - if you have speakers that 'make your music sound amazing' in your studio by say, boosting the bass a bit. When you pass that music onto others, their system will have no such bass boost, so your music might sound thin and crappy.
Look into the concept of MASTERING if you are curious to learn more.
PS. I have a pair of HD25s which are comfortable and have good sound exclusion. I use them for music listening, studio, and DJ applications. For actual mixing I prefer speakers - monitors which I know the sound of very well.