CI stands for continuous integration, CD for continuous delivery or deployment. What is meant is a process that starts on its own as soon as somebody changes something: the application is built, the tests run, the code checks run, and only when all of it is through does anything carry on.
The benefit lies less in the time saved than in the reliability. A check somebody has to start by hand gets skipped under pressure. And you are under pressure exactly when something needs fixing quickly, which is the worst possible moment.
Shipping without downtime
The second part concerns the route onto the server. A deployment during which the application is gone for two minutes is unacceptable in many places. The usual route: the new version is built up completely alongside, and only once it stands does the switch happen.
That involves a few precautions you take once. Changes to the database have to be cut so that the old and new versions can run side by side briefly. And there has to be a way back that does not consist of hunting a fault under time pressure.