Dolby Atmos has "audio beds" and "objects".
Audio beds are channels, just like the ones in 2.0, 5.1, 7.1 etc configurations.
Objects are sounds that are independent from channels. There can be up to 128 objects simultaneously.
Now let's assume that a soundtrack is composed of a stereo acoustic guitar, and a mosquito flying around in a room.
The acoustic guitar will be mixed as "audio bed". This means that it will be heard through the front left and front right speakers, no matter how many speakers the system is composed of.
The mosquito flyes across the room. As it moves, it will be heard from the speaker(s) that are closest to the position of the mosquito in the room. If the mosquito flies in every corner of the room, it will be heard in turn from every speaker. If there are 2, it will be heard from 2. If there are 9, it will be heard from 9. If there are 64, it will be heard from 64 speakers. So, one single object can be heard from up to 64 different speakers.
Now the mosquito exits the room, and 128 bees enter the room, but they fly close, and they stay near a corner. Each bee has been recorded separately, and each bee is an audio object. As they stay in the same place, they will be heard from just one or two speakers. So, 128 different objects can be heard from just one or two speakers. Should they spread across the room, they will be heard from all the speakers.
Does any of this make sense to you? Audio beds are just channels. They are the static part of Atmos. The audio bed part will go to fixed speakers, no matter where they are.
Audio objects will go to those speakers that the Atmos decoder deems as the closest to where the audio object should be.
That is why even if you don't have ceiling or height speakers, even if you have a 3.1 system, you will benefit from Atmos. The audio objects will be, in every case, be reproduced by the best speaker.
(This post is intended just as an idea to help all those AVS members who seem to be struggling with these new concepts. I hope someone will write something a lot better that what I did. English is obviously not my native language!