Imagine if you are running on a conveyor belt at 60 steps per second. This belt is turning a crank that, let's say, moves water. You are the energy input (the sun for solar, heat energy for steam turbines, movement of wind for windmills, and so on), the crank is the generator, and the water is the load.
Now imagine the water is suddenly turned into vegetable oil (more load has come on). It is more difficult to move. For a brief moment, your energy input is still the same but you can't push as much of the new liquid so your pace slows down a bit, to 58 steps per minute. You increase your exertion (add more energy input) and your pace climbs back to 60 steps per minute.
Finally, imagine that you're running as hard as you can. That 60 steps per second to push that vegetable oil is as much as you can do. But the vegetable oil turns to heavy cream (even more load). You've put in as much energy as you can but your pace falters and drops to 49 steps per second.
For the same reason a car engine’s RPMs will drop when you start going up a hill (unless you increase the throttle).
Spinning up the generators, and keeping them spinning, takes energy on top of whatever the load is. If the load increases, but the input energy does not, the difference is robbed from the momentum of the spinning mass. This slows the generators down thus the frequency drops.
ELY5: When you're at a playground, and you're pushing the roundabout round and round, it gets harder and you get slower the more kids jump on it, right?
ELY not 5, but I'm no expert either: in terms of phase sources like generators lead [0] sinks, by an amount proportional to their over-production; so the more sunk, the less they lead, and the lower the frequency.
