A warm up is typically 5 to 10 minutes of light exercise, most often recommended on a cardio machine like a bike, to transition the body from a less active stage to a more active stage, which will make your body more receptive to the stress/strain of your regular workout.
Some individuals prefer to do their abs before other body parts, but that doesn't mean it's necessarily a good thing for you. All of our bodies are different. You should view your abs as another muscle group to train just like you would chest, back, etc., instead of something to throw into a warm-up. Additionally, if you just go and do some ab exercises, your body might not be ready for the exercises, and you run the risk of injury without a proper warm up.
Lastly, Abs generally refers to the rectus abdominis and this muscle does not come into play in every exercise. It's highly recommended that you focus on all of the major muscles of the core region and not just the rectus abdominis. Therefore, a balanced core workout is much more beneficial for providing core strength and stability, than blasting out some abs exercises before you go and do the rest of your workout.