Link building involves doing certain activities to encourage other websites to link to your web pages.
Backlinks from other sites tell Google that that site is willing to send someone from their page to yours. This means that the website must trust you enough to potentially risk their reputation with that user.
Backlinks should exist for the purpose of providing extra information. So whenever links are being built, they need to be in relevant places that make sense for the user.
Acquiring more links to your website in general is beneficial, as if many sites trust you with their users, Google believes they may also be able to trust you with their users.
However, be careful! Google doesn’t like you building links that don’t occur naturally, or don’t serve the user. If you screw this up, it’s quite possible you’ll be punished in the rankings.