It's a seamless integration tool. This is a methodology that preaches that everything that is being done about development must be integrated immediately, usually several times a day. So you have the centralized project where all developers are putting their modifications where the system is tested and built completely showing if something is going wrong and identifying problems as soon as they appear and not later when it can be more complicated.
When I speak in tests I speak of a set of quality checks and not just unit tests.
In some cases the implementation itself is simulated to make sure everything is ok.
Some people question how useful this is. There are those who demonstrate that there are difficulties in operating this way.
Obviously, in small teams the utility becomes much smaller, but it is still possible to benefit from the automation of the complete quality assurance process.
Travis CI takes care of all of this (if properly configured for each task, it does not guess anything, gives a job all the time leaving everything in order, but can save a lot of work as well) and it communicates with several other tools that support the development of team software. He alone does nothing.
It controls the flow of the source repository, the build, the various unit, integration, load, performance, static and dynamic tests, creates documentation, simulates or even prepares the bug , eventually even opens tickets .
A competitor talks about CI in Portuguese .
IBM Article .
About Travis CI .