Something that's spotty is uneven or inconsistent, especially in quality. If you can never rely on your internet connection when you're trying to watch a movie, you can call the connection spotty.
The spotty reporting of your local newspaper means that every once in a while it publishes a well-written, newsworthy article, but often it's not worth reading. And your spotty attention during biology class probably means you'll need extra review sessions before you take the final exam. You can also use this adjective to mean "spotted" or "covered with spots," like your spotty Dalmatian puppy.