These are specially crafted events with a set amount of races that might remind you of Career Mode. Every Series is designed around a theme (brand, class, country of origin) or car. As usual, the higher the stage, the better the rewards. Sometimes limited time series appear that are tied to seasonal or real-life events, such as the Christmas Series or the Oscar Series.
Each Series has its own requirements for the brands and models that are allowed in the Series. You can win Wild Cards, different kits, and gold rewards. Keep in mind that once you’ve completed a race in the Series or the Series itself, it’s over and done, and you have to move onto the next race or wait for the next Series.