It is indicating how long it takes for transistor to turn on or off, since transistor does not switch instantly. But this number is only for specific situtations. YOu must look at other parameters to calculate rise/fall time for YOUR situtation (for MOSFET, total gate charge is very important for rise/fall time).
Faster rise fall time makes more noise and voltage spikes but lower heat. Slower rise/fall time makes less noise and less voltage spike but more heat. Heat is caused by being halfway between the full on and full off state. Full on and full off are efficient and dissipate less heat than halfway between.