I have to give you a two-part answer. First, the reviewer is using a metaphor. He is talking about a Netflix movie, but he is using descriptions that apply to food. This is common. For example, someone might call a comedy a "delicious confection," although literally a confection is a sweet food like cake or candy. Second, let's move on to food. A "treat" means a food that gives special pleasure--perhaps something eaten rarely, on a special occasions, as a reward. "For the big Thanksgiving dinner, my wife fixed all of the family's favorite treats." "Palate" and "cleanse" are words used by people with a serious interest in food, such as professional wine tasters. "Palate" has a double meaning. It literally means the roof of the mouth. In language learning, it's used in phonology, in diagrams and descriptions of how you form the sounds of speech. When talking about food, it means the sense of taste generally. When a professional taster is comparing differences in flavors, he won't follow one food immediately with another. If he did, it would be hard to taste the second food properly. Its flavor would be mixed with leftover flavor from the first one. In between tasting, he might "cleanse his palate" with a sip of tea, or a slice of apple, or something like that. Someone giving a very serious formal meal might include courses in between the main courses to "cleanse the palate." The reviewer is comparing an evening of television to a feast of food. He says that "The Umbrella Academy" is good, but simple and bland, and that, like a palate cleaner, it would be nice to watch in between two more intense shows.