the on-resistance (R_DS(on))

When a MOSFET is fully switched on, the channel from drain to source is not a perfect short: it has a small but real resistance called R_DS(on), the drain-to-source on-resistance. This single number tells you how good the switch is. A modern power MOSFET might have an R_DS(on) of a few milliohms; older or smaller parts have tens or hundreds of milliohms. The lower it is, the less voltage the device drops and the less heat it makes while passing load current.

The heat follows the resistor power law: a MOSFET conducting current I dissipates P = I^2 times R_DS(on). For example, a part with R_DS(on) = 10 milliohms (0.01 ohm) carrying 10 amps drops 0.1 V and burns 10^2 times 0.01 = 1 watt as heat. Halve the on-resistance and you halve that loss. This is why power-switch designers obsess over a few milliohms: at high current the I-squared term magnifies every milliohm into real watts and real temperature rise.

Two honest caveats. R_DS(on) is not fixed: it depends on gate drive (it falls as you drive the gate harder, which is why partial gate drive is so lossy) and it rises with temperature, typically increasing by roughly half again from 25 C to 100 C. So always size your heat budget using the HOT on-resistance at your actual gate voltage, never the rosy headline figure from the datasheet's first line.