Testing is an important aspect of any software project. For backend software and components, this will typically comprise of automated tests, but also in a UI application it will involve manual testing and interaction with the UI.
In spite of how important it is, testing is often a neglected area of development because of how inefficient it seems. In a large system, you can spend a lot of time waiting for the application-under-test to restart with your fresh changes so that you can test them. The result is simply that we don't test as often as we should.