Raconter des salades directly translates as telling salads. Is it about diets or healthy eating? Far from it; it is one of those French expressions you can't make sense of based on the meaning of the words alone. It means telling elaborate lies, or spinning yarns. Not a small, insignificant lie, or a white lie, but rather a big lie, or a whole bunch of them, all tied together in a story, maybe even mixed up with some truths so as to appear more plausible. It is a colloquial idiom you will hear often in every day language. It is the equivalent of the more prosaic raconter des histoires - telling stories.
The use of salades as a metaphor for lies appears in the 19th century. Why would one tell salad tales to spin a yarn? Presumably because a salad is a variety of colorful ingredients that go well together and make an easy-to-swallow side dish. Similarly, lies, tall tales and outright fibs mixed up with some facts and marinated with half-truths are more likely to pass for real, especially if you season your story with a convincing tone of voice and serve it up as fresh and healthy.
Note that in French une salade may be a dish, like a salad in English, or it may just be a lettuce or any other type of raw, leafy greens. The French translation of the English "salad" is une salade composée, or the actual name of the salad such as salade grecque, salade niçoise, salade paysanne etc... Mixed greens, on the other hand, will translate as une salade verte, or simply une salade.
Arrête tes salades !
Stop talking nonsense! Je te jure que ce ne sont pas des salades ! C'est exactement comme ça que c'est passé.
I promise I'm not lying, that's exactly how it happened!