Continuous integration (usually abbreviated as CI) is a series of software engineering practices that ensure that code is always in a working state.
The term continuous integration comes from historically having to integrate software frequently, often multiple times a day. This arose from the fact that developers worked with local code that was not necessarily joined with other people's code automatically. Nowadays, using a source-control versioning software such as Git makes some of the elements automatically available.
Continuous integration emphasizes on having potentially releasable code at all times. This makes releases possible very often, with small increments of code.
Making more releases more often actually generates an increase in the quality of each release. More deployments also mean that each deployment is smaller...