Okay, here's a silly explanation.

Imagine two people want to communicate. They shall be named Alice and Bob. It's night, and they're too far away to hear each other; but each has an electric torch, and they can see the light coming off the other person's torch.

Those torches are fancy. They have two settings: "strong" (big arse blinding light) and "weak" (wee light, but still visible). Let's call a strong flash "1" and a weak flash "0" for short.

They also have a code, that they use to interpret the flashes of light that they send each other. Here it is; check the "binary" column. For example, if Alice sent Bob a weak flash, then a strong flash, then weak, strong, weak, weak, strong, strong, then Bob is supposed to interpret this as an "S".

This is already enough to communicate. Like this:

• Alice: 0000 0010 0100 1000 0110 1001 0010 0000 0100 0010 0110 1111 0110 0010 0000 0011
• Bob: 0000 0010 0101 0011 0111 0101 0111 0000 0011 1111 0000 0011

Remember, each "0" is a weak flash and each "1" is a strong flash. When we decode it with the table from the link, here's what you get:

• Alice: Hi Bob
• Bob: Sup?

Now. Messing around with the settings of a torchlight is slow, painful, and annoying. So is to watch closely for light flashes, write them down, and decode them with a table. So... let's use machines to do so?

• Encoder: a fancy machine with a built-in torch. You type "S" down that machine, and it outputs weak, strong, weak, strong, weak, weak, strong, strong flashes in this order.
• Decoder: another fancy machine. It has an optical sensor; if it gets the exact sequence of flashes from above, it'll output "S".

Okay, now let's replace the torch. We want something that emits light that you can see from as far as reasonably possible; like, from Mars if you want. We could instead use light that has been amplified by radiation, it's really strong and directed. The common name for that is "laser".

The simple way is like Morse code, variable gaps and lengths of emissions to convey information.

The more complex version is that plus variable intensity and variable wavelengths for a more complex "language".

Using light to communicate has a pretty long history, you just blink lights in a pattern, same as electric signals just without a wire, I mean you can use a "wire" to make light go further (optical fiber), you send zeros and ones by turning the laser on and off, that's all, optical audio connections came to the market in 1983, the undersea internet cable are optical etc, you can blink a flashlight in Morse code and it's a light based communication, swap flashlight with a laser and you have "an information in a laser beam", that's all, you want to send more info faster just blink faster

That’s how fiber optic cables work now.

You can use a laser for line of sight through the air, so you don’t have to lay fiber. So ship to shore

How it is transmitted, others in this thread have said, light on is a 1 off is a zero, now just transmitting information in binary. You can go as fast as you can reliably turn it off and on, and understand the info at the other end.

The real trick is using multiple wavelengths at the same time. So your data goes at 100nm mine at 110nm and yes that is a gross oversimplification.

Digital data is just 1s and 0s.

On and off. If you turn that laser on and off you're transmitting binary information. The receiver can interpret that. We already do that with fiber optic cable, basically. This is just removing the wire.

As the other comments stated, it can be as simple as turning the light on and off and we already do this with fiber optic network connections. The light particles themselves aren't carrying the data, just the pattern of on and off.

It is also possible to send multiple streams of light using color filters. It's called Wavelength Division Multiplexing.

you can pulse it or encode it changing color. this is nothing new.