Link Building is when you increase the number of inbound links to your website to increase your sites search engine rank.
It is the act of getting other websites with similar keywords to you to link back to your site. This makes search engines see this as an ‘act of confidence’. This is the websites saying this is also a good source meaning that the search engine will recognise that and rank you higher.
If your site has lots of other sites linking to you it shows that a wide range of people, consider your site to be a good source of information. If all these sites have similar keywords to you it will also show that your site is very accurate when it comes to your Keyword.
This will increase your position on search engines ranks and increase the number of visitors you have to your site.
The best way to link build is either to create great content and wait for other sites to link to you. Or be bold and reach out to other websites and ask them to link to your site. This can be daunting, but it can improve your sites rank significantly.