The sentence contains two metaphors which describe things that politicians do.
You get a few potholes filled means you do accomplish a few minor administrative things while you're in office. Potholes are all over the road and continue to appear all the time, so it's just an ongoing maintenance task. There's nothing great about filling potholes.
Keep the lid on as best as you can refers literally to a container that has a lid. What's inside the container is something you want to keep inside. Metaphorically it just means you are trying to keep things organized and prevent things from spilling or falling apart. In the context of politics, it probably means you are keeping the day-to-day bureaucracy running while trying to avoid public tragedies.
Neither is particularly impressive, and the person saying the line is just trying to emphasize how dull and pointless it is to be a politician. Unless you're Donald Trump.