What are "pick-up", "dropout" and "rated" voltages in a Relay datasheet?
Rated voltage is the coil voltage that particular model is designed for. (The first page of the datasheet shows that the last digit of the model number is the coil voltage.)

As for the others, the "pickup voltage" shows that the relay is guaranteed to not require more than 70% of the rated voltage to switch the contacts "on". It may turn on with less voltage but this is not guaranteed.

"Dropout voltage" shows how low the coil voltage must go to turn "off" again. It will always turn off at 10% or lower. The value may be higher (though not as high as the rated pickup voltage.

So, to put it another way, if you are using a 5 volt model, and you make sure that the on voltage fed to it is at least 3.5 volts (70% of 5v) and the off voltage is no higher than 0.5 volts (10%), the relay will work as you would expect. If the on voltage is lower or the off voltage is higher, it may or may not switch.