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 Rds(on) is important in switching applications. Knowing at which voltage you can attain that Rds(on) 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 a Rds(on) of no more than 10mohm, you could search for those parameters. But if that Rds(on) is attainable only at 10V, while you only have a 5V powered MCU with no other power rail, you know that MOSFET is not suitable (or will need additional circuitry to be used).

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_DS(on) is important in switching applications. Knowing at which voltage you can attain that R_DS(on) 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_DS(on) of no more than 10 mΩ, you could search for those parameters. But if that R_DS(on) 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).