Continuous integration originated with Extreme Programming. Developers integrate their work frequently (at least daily) with a shared mainline (this assumes you're using source control), to avoid the "integration hell" of attempting to integrate a large amount of work.
This practice has now been adopted far more widely. It typically requires a build server which creates a build in response to a code check in, and then runs a suite of automated tests against it.
Martin Fowler's article is a good read: http://www.martinfowler.com/articles/continuousIntegration.html