It is the practice of ensuring that all aspects of your software development process are lined up to permit the daily creation of a working version of your product. It is best known as part of Extreme Programming.
This involves things as far afield as build automation, automated testing, daily check-ins, using a source code repository, etc. But the ultimate goal is to help the entire project run according to core Agile Principles so that you deliver early and often. This, in turn, helps you leverage feedback from your users, etc.
