Continuous Integration is a software development process where the code is continuously tested after a commit, to ensure there are no defects/bugs.
In large teams, many developers work on the same code base. Thus, any multiple commits can cause a bug. With continuous integration, bugs can be identified early and fixed before pushing changes to production. Any new code is integrated into one executable form, termed a build. If the build is green (i.e. all ok, no bugs), then the executable artifact can be deployed. If not, the bug needs to be fixed, and the new build is tested again.