The ring group refers to an extension number, which if is dialed will ring multiple phones/extensions at the same time or after each other.
Usually will ring all phones, until one of them will be picked up.
For example: you can create a group of five technical support guys and assign the 9999 extension number to this group.
When the customers will dial 9999 will ring five phones at the same time and they will stop to ring when one of them will answer.