So, you have these packets of data – like boxes of cereal, man. Some cereal boxes are big and like some are small man. So if people have a smaller bowl, a smaller box of cereal can fill it up. This is how it works with video data or “bits.” A smaller cereal bowl (slow internet connection) will take less to fill it, while the big bowl people will eat more cereal and have better video.
Adaptive Bitrate is like having a smart transmission for your car. If your viewer can take more data and a higher quality stream, then adaptive bitrate adjusts it automatically. Otherwise you would have to guess what gear will work best for everyone and use only that gear.