Not for sure what you really are asking. When you connect a generator on line to an electric system, you have to make sure it is in sync with the system you are connecting to. You use a synch check scope or meter to check between your generator and system before you close the breaker to tie in the generator. The generator is usually connected to a steam turbine. Here in the USA the turbine will be rotating at 3600 rpm, called synchronous speed.
There are a couple of points here. One is mentioned above. The other is the type of generator. A synchronous alternator is designed to generate power at a given frequency when rotated at a given speed by some external means such as a steam turbine or diesel. It is separately excited via DC and can provide its output alone (no need to be connected to the main grid). This is in contrast to an asynchronous generator such a a double fed induction generator often used in wind turbines which must be connected to an established grid in order to be able to generate power and will run slighlty above the synchronous speed by the slip necessary to deliver the required power.