Definition: the practice of frequently integrating one's new or changed code with the existing code repository -Wikipedia
Merging new code into master often sounds awesome, but we've been learning the value of testing and the importance of a passing test suite.
But, as your projects grow, your test suite should grow as well. We're all lazy and forget to run the entire test suite everytime we create a new commit. For large projects, running the entire test suite can take hours. So we do what all lazy people do, make a computer to the work for us.