This is a very common problem that is easy to fix if you are just getting started with time-lapse photography.
Please make sure that all of your camera settings are set to manual (auto-focus off, white balance manual, aperture manual, shutter speed manual, ISO manual). This is so that settings do not change between pictures. It is important that your camera does not try to adjust for changing light conditions or moving subjects during the time-lapse. Please see the "Camera Settings" section of our Guide to Time-Lapsing for more information.