What is a Nightly Build?
A nightly build is a version of the final software for each night, it's not a beta version or something, it's more recent, a version that is being working on, when a project is big and have many developers contributing to make some software, it could be hard for each of them to compile on his own and merge their work, so they work little by little and over night a server compile the whole work they done together and merge it into a version that they can test, that build (version) is a nightly build, so (it's done over night, it helps developers on large projects, usually it compiles it self by a server, and it help developers when they are on different time zone).
I hope I helped and you can pretty much find more on Google
Also from Wikipedia
A nightly build is a neutral build that takes place automatically. These typically take place when no one is likely to be working in the office so that there are no changes to the source code during the build. The results of the build are inspected by the arriving programmers, who generally place a priority on ensuring the recent changes to the source code have not broken the build process or functionality of the software.