A link is something like a recommendation for your site and the most important means of external optimisation. The more links leads to the resource, the better it is perceived by the search engine. But there are subtleties. Some links are important, others are useless and even harmful. Instead of a hundred meaningless links is better to place one good one. Moreover, incorrect work with link mass can lead to sanctions from search engines. Control of the process in "Yandex" is engaged in the algorithm "Minusinsk", in Google - "Penguin".
How to check whether the link is of high quality? This is done in different ways. It is important that it leads from a thematic resource. For example, if you promote a site about the sale of flowers, it would be strange to put links to it from a resource for the sale of concrete structures. But the site of a wedding salon will be an excellent donor.
Attention! Some unscrupulous owners of competing sites can use unfair techniques to harm your resource. For example, they can purchase a large number of links to your site from bad sites. Is it possible to combat this? Yes, search engine tools are designed for this purpose. In the webmaster panels of Google and Yandex has the ability to view the links that are known to search engines. You can also use third-party services, such as ahrefs.com, to analyse incoming links to the site.
To avoid trouble and achieve a really high-quality result, it is better not to waste time on independent work, but to turn to AVSEO experts.