Backlinks are links from other sites to your own site. This is not something you can create yourself, but that other site owners will create on their own sites if they want to link to your site, if they find it interesting or valuable. You can promote your site to other site owners to get more back links, here is guide to help you with this: https://moz.com/beginners-guide-to-link-building
Related questions:
> How and who can create back links for my website?
> How to build links to my site?