If you are driving a boat, and the coast guard pulls up alongside and commands THROTTLE YOUR VEHICLE! through a cone or a bullhorn, you probably should. You probably should slow down. That means ease off the gas. That means... throttle (reduce) the fuel injection lines rate of delivery of fuel to the engine.
If you get told to slow down when you're in an un-motorized sailboat... Well, you better just do something with the sail or whatever.
There are streams of data running through our intertubes... Our internet is conceptually a set of tubes through which streams of data flow.
When you throttle a tube of data, it means that you aren't letting as much of the data flow through. It's like you are constricting a pipe.
The owners of the pipes are companies like AT&T. They own the fiber. Or Comcast. They own the cable. Our internet flows through their tubes. But there are other companies that send their data through the same tubes.
The tube owners are utilities. So when Time Warner tries to send it's customers data through Comcast's pipes, or whatever... Well, if Comcast were throttling... or if AT&T were throttling: That would mean it is AT&T's fault that the Time Warner runs slower than it should.
Hypothetically speaking, of course. I don't know who owns the pipes these days. I just know that
it is the pipe owner's fault
they do it on purpose to make their service look better than their competitors who use their pipes