A backlink is a link that connects a page or web resource to another.
It could be internal or external.
An example of an internal backlink is this
An example of an external backlink is this
How important are backlinks?
According to Google, backlinks are part of their top 3 ranking factors that they use to rank a website on SERP.
The remaining two are high quality contents and search intent.
Google places weight on Backlinks because they act as votes for websites. Though this isn't a numbers game where the one with the highest number wins...nah. Backlinks that move the needle are far from that. This is a game of quality where sites with quality links win.
There are several ways to build links but not all move the needle. Some are legit while others are not legit and others are in between.
Backlinks are also important because they form the basics of how Google is built - Page Authority.
So, would you rather have 10K low quality sites link to your site or 5 well established sites with high authority link to you?