To complement what Stefan Wyss correctly said in his answer, I'll give you the rationale for that parameter.

Power MOSFETs are often used for switching, that's why a low R<sub>DS(on)</sub> is important in switching applications. Knowing at which voltage you can attain that R<sub>DS(on)</sub> is an important parameter because you can tell immediately, without looking at the curves in the datasheet, if your circuit could drive that MOSFET fully on or not.

For example, if you need to switch a 10A load with an R<sub>DS(on)</sub> of no more than 10&nbsp;m&Omega;, you could search for those parameters. But if that R<sub>DS(on)</sub> is attainable only at 10V, while you only have a 5V powered MCU with no other power rail, you know that that MOSFET is not suitable (or it will need additional circuitry to be driven).