In short, the main principles of DevOps are automation, continuous delivery, and fast reaction to feedback. You can find a more detailed explanation of DevOps pillars in the CAMS acronym.
DevOps requires a delivery cycle that comprises planning, development, testing, deployment, release, and monitoring with active cooperation between different members of a team.
The main reason to implement DevOps is to improve the delivery pipeline and integration process by automating these activities. As a result, the product gets a shorter time-to-market. To achieve this automated release pipeline, the team must acquire specific tools instead of building them from scratch.