It uses the 2.4 GHz band
Does this refer to the frequency of the EM waves that the device uses to communicate?
Yes. It generates (and receives) radio waves in the region of 2.5GHz.
If used in open space and with lower baud rate its range can reach up to 100 meters.
I know baud rate refers to how many "groups of bits" are transmitted per second. I don't quite understand why the range increases for a lower baud rate.
Think of it like talking to someone. If you're stood right next to them you can speak nice and fast and they can understand you. Your speech comes across nice and clear.
However if you're the other side of a quarry you have to shout really loud, and if you talk too fast they can't understand you. You. Have. To. Shout. Slowly. so that they can understand you.
The environment makes a lot of difference to how your speech propagates. In large areas there's lots of echo and other things. It's similar with RF communications. There's more noise and things that can interrupt communication. By having lower baud rates you're giving the receiver more time to understand what it's receiving and make sense of the data.
The module can use 125 different channels which gives a possibility to have a network of 125 independently working modems in one place. Each channel can have up to 6 addresses, or each unit can communicate with up to 6 other units at the same time.
Here, does a channel mean a single frequency? How is the range of possible channels decided (why 125) and how is the smallest detectable resolution decided (smallest change in frequency which would cause a change in channel)? So does a single unit work on only one frequency or could the same unit communicate at different frequencies at the same time and so be a part of different networks simultaneously? In effect, I am not able to make sense of this paragraph!
There's a number of different factors at play here. First there is the "Bandwidth" of the channel. This is the maximum amount of data that can be squeezed into a particular carrier frequency with a particular amount of "space" around the signal. You can think of the channel width as the diameter of a pipe. The wider the pipe the more water can be pushed through the pipe per second.
You want to slice the RF allocation up into enough channels to give enough concurrent access to lots of devices, but at the same time you want to have enough bandwidth per channel that you can transfer reasonable amounts of data.
It's all a balancing act.
Nordic decided 125 channels was the optimal balance for their application.
One nRF24L01+ can work at one frequency at a time. That's not to say you can't instruct it to change frequencies at any time, but it can only be listening on the one frequency at once. Just like a normal FM radio - you can only listen to one radio station at a time.
But the nRF24L01+ protocol uses addresses within that one chosen channel, so lots of devices can all be on the same channel to talk together, and each is distinct by its address.
The protocol also includes the concept of "pipes" which are virtual wireless circuits between two physical devices. Once you have configured a pipe with an address any other device can communicate with that pipe. You can configure up to 6 pipes, so up to 6 devices can send to the same target at the same time and each devices data is kept separate and distinct from the others. It's kind of like having 6 serial ports all connected to different devices at once.