What is travis-ci ,software build ,tests?

I recently was learning various tools to get started to open source contribution. I found this tool called travis-ci which said that its used for testing,now i am further confused that what is testing(how and when it is done). Another thing is build,what is software building ,is it like compiling the code?,and when and where is the software building in an open source contribution process.Please someone clear these topics because these are delaying me in getting started for open source contribution.