A nightly would be a nightly build version of a release (`1.0.0+build.12345`) or of a pre-release (`1.0.0-alpha.1+build.12345`).  You can combine prerelease and nightly in different ways to affect how bleeding-edge you want to be:

prerelease = false, nightly = false => stable releases
prerelease = false, nightly = true => nightly builds of a release
prerelease = true, nightly = false => official pre-releases (alphas, betas, release candidates, etc.)
prerelease = true, nightly = true => bleeding edge, we-just-wrote-this-code-today