When a new piece of content is created, it is assigned a sequential numeric ID known as the node ID, or NID. By default, the URL displayed when the content is presented, similar to http://mysite.com/node/123, where 123 is the NID. If you want your content to have a more meaningful URL than node/123, such as sasha-discovers-donuts, that custom URL, known as an alias, can be defined here.
When an alias is defined for content, the original URL will still be available for use. You will be able to reach this content using either form of the URL, e.g. https://mysite.com/node/123 or https://mysite.com/my-friendly-content-name.