A stack is a collection of blocks. With this stack, you can make a collection of blocks which you can show on multiple pages. No need to make the blocks over and over again. Usefull for a sidebar with a twitter feed, news feed, share links all together, which may be the same for a lot of pages. You can come up with loads of examples, this is just one of them.

Think about it as sort of a "Global Area", but without it being an area.. but a stack :)

Thanks but if I have a stack and a content block that says ABC and I have that stack on 3 different pages and I want to change one of them to abc that will change on all 3 pages right, not make a separate instance of it?

Correct. A stack is meant to display all the same content on each page you use it. So if you make a change to a stack, it will be visible to all pages you use this stack in!