An example of a natural backlink would be like when a person is talking about a topic on a forum and they link out to a page that serves as a resource of the information about what they are talking about. Like if I told you to read about how to get
Google to crawl your site here, that is a natural backlink because I am showing you a page on Google that I recommend you read about that topic.
It is natural because I am not affiliated with Google in any way. I am simply referring to that page because I think it is useful.
So, if you had a website that had useful information on it and people are creating links to certain pages on your site like that because those pages are useful then those links would be considered natural backlinks.
If you have a friend go onto a forum and create those links for you or if you do it yourself then those links are not really natural. You might be okay doing that anyway, but technically they are not natural links. Any links you create for the purpose of promoting your website are not really natural links. There is a bit of gray area there though. Because if someone on a forum publicly asks me what my website is, then it seems entirely logical that I could link to my own site. And in those cases you are typically okay as long as it doesn't look like you are going all over the place and faking that.
Another example of a natural link would be when you link to your website from your Twitter page or Facebook page or any social media account. Google expects you to do that because that is normal.
