There are some blocks that you use them in more than one pages and you want them to be identical in every page. For example the Header or the Footer block. So let's say that you create two pages and you put the header block in both. You go to live editing and you change the logo image. The logo will change in that page but not in the other one.
The solution is to make the block Shared. Create your first page and go to live editing. In the block's options box you can see the MAKE SHARED button. Click on it and make the block Shared. Now create your second page. Click on the Add New Block button and when the LIBRARY panel opens up, click on the SHARED button on the top of it. You can see that the list of blocks changes and it now shows your shared blocks. Click on your shared header block to use it. Now whenever you make a change to that block in one page, all pages that are using it will be updated.