Yep, that's it. Basically, even your highest quality hifi seperate speakers have troughs and peaks in their frequency response. Sometimes they're engineered this way to disguise shortfalls in construction, or to make them sound louder, and sometimes its just cheaper components.
Monitors are smooth and clear. A decent set of powered monitors have a different amplifier built into the speaker for EACH component. So if your monitors are made of a tweater and mid/woofer, then each has its own amplifier. The amplifier is engineered as a perfect balance for its relative component, and the component is engineered to precisely reproduce a certain range of frequencies. All components complement each other.
To cut a long story short, frequency response on your average speakers is a bit up and down, with cheaper speakers having clear pockets and peaks for different speakers. Cheaper monitors have similar problems, but a decent set produces all frequencies evenly across its spectrum. As a musician, if you want to move to that next level in production, making the track sound just right across the frequency spectrum, monitors are the way to go.
Insiiiiiiiiiiiiiiiiiiiiiiiide!