Basically it allows you to chat in one text window with like 10 of your friends. Without group texting, when someone replies to your mass text, they come back as single text and will only send it to you specifically so everyone else on the list can not see their text. So think about it like this. Group text allows you to setup a chatroom with a bunch of your friends. Everyone can see anyone's reply all on one window.
Also, with a lot of iPhone users, they will include you in a group text but when you reply, it only goes to the person that sent the text first and not to everyone else on this list.
Now imagine if you were included in a group text with 10 people. When you reply, in order for everyone to know what your response is, you have to message individually everyone on that list. With group text, you reply once in that window and everyone gets it. It