Wikipedia has it that Yu Sang is a Teochew-style raw fish salad. It usually consists of strips of raw fish (sometimes salmon), mixed with shredded vegetables and a variety of sauces and condiments, among other ingredients. Yusheng literally means "raw fish" but since "fish (鱼)" is commonly conflated with its homophone "abundance (余)", Yúshēng (鱼生) is interpreted as a homophone for Yúshēng (余升) meaning an increase in abundance. Therefore, yusheng is considered a symbol of abundance, prosperity and vigor.
My family and I tossing Yee Sang on the first day of the Chinese New Year.