A backlink is an incoming link (in a web page). The backlinks of a web page are all the links that "point" to this page, whether these links come from external websites or internal pages.
Importance of backlinks for SEO
To be schematic: the more a web page receives backlinks (from pages of "good quality") the higher its "popularity" will be, the more it has chances to be well referenced. The number of backlinks is only one criterion among others.
To be more precise... In 2019, it is more the "quality" of backlinks that is taken into account than the quantity of backlinks. To optimize your referencing, the objective is to try to get backlinks from sites of good reputation (a backlink from Lemonde.fr will have much more impact than a backlink from the unknown blog of your little cousin) and that deal with a theme close to yours.
Backlink: the text of the link (its "anchor")
The text of the link (the text on which one clicks that we call "anchor" of the link) is very important for referencing. Ideally, this text should correspond to the keyword of the target page.
For example, if the link points to a page that deals with recruitment agencies, it is preferable to have a text such as :
"discover the know-how of our recruitment agency"
"to discover the know-how of our recruitment agency, click here" (the words "click here" do not help your referencing).
To optimize your referencing on a specific keyword, it is therefore preferable to use text links (hypertext links) rather than links located on images.
Textual anchor: beware of over-optimization
You should avoid having always the same text anchor for all the backlinks that point to a page. This would look suspicious to Google.
You need a "rotation of anchors" to make the "netlinking" look natural. If you are looking, for example, to optimize the referencing of your page for a keyword such as "recruitment agency Paris", you must try to get links (backlinks) on anchors such as "recruitment agency Paris", of course, but also "recruitment agency", "recruitment agency in Paris", "recruitment specialist", as well as not optimized anchors at all, or even just the URL of the page.
Backlink: the ideal page to create a link
Ideally, the original page (the page where your link is located) :
deals with a theme consistent with the target page (to stay on the example of the recruitment agency, it is preferable to create a link from a site dealing with employment, HR ... a link from a blog of cooking recipes would bring nothing, or not much)
has a good reputation, a good "popularity index" (the Page Rank of the original page.... even the "Page Rank" is no longer, for a long time, communicated by Google, we can base ourselves on indices of the type "Trust Flow" provided by the Semrush tool).
does not have too many outgoing links
Hard links and Do follow
To have a favorable impact on your referencing, the links must be "hard" (direct link that does not go through a redirection script) and in dofollow. Indeed, some links can be coded in " nofollow " and, in this case, they do not allow to optimize the popularity of the target page.