What is the meaning of views displays?
Views displays helps you manage your views that are similarly the same.

Basic example is articles:

On your home page, you want a simple block that shows 5 articles, no pager.
At the bottom of this block, you want a link "read all articles".
It redirects you to a page with all the articles, 10 per page, with a pager at the end.

Instead of creating 2 views, which is a waste of time, you'll create one view with 2 displays.

For this example, you will create two views displays, one "Block" and one "Page".
First you create your "Page" display (or "Block", as you want), set it to filter only articles content type, show the title and the date of publication.
Then you can create the other view display, and you only need to override the "Pager" settings.

Tomorrow, if you add an image to your articles, and you want them to appears on both the block and the page, you just need to add the image in the fields, and it will appear on both views display.

Beware that each views display has its own purpose, for example "Feed" will all you to create Rss display, "Page" needs it's own URI, etc.