さしすせそ is all the letters in the S column but in cooking さしすせそ has a special meaning: that is, to describe the correct order to add various seasonings. You should add the following ingredients in this order.
さ is for さとう (sugar) and さけ (sake, Japanese rice wine)
し is for しお (salt)
す is for す (vinegar)
せ is for せうゆ（the old spelling for しょうゆ, soy sauce)
そ is for みそ (miso, fermented soybean paste).
As sugar and sake have bigger molecules, adding them first prevents food from falling apart during boiling. Also they make the ingredient go soft quicker.
Salt takes water out of the ingredients. So add salt after the ingredients get cooked enough.
Vinegar, soy sauce and miso are all to enjoy 香り（かおり – fragrance), so it’s best to leave them till the last and among these three, vinegar is the strongest, so vinegar goes in before the last two.
The fregrance of the miso is the most volatile, so miso goes in last. If you are making miso soup, it’s best not to let the water boil after putting miso in so that the maximum fragrance is kept in the soup.
Last week, I talked about mirin and mirin style flavouring. It is recommended, the real mirin should be used together with sugar and sake but the mirin style flavouring should be added after miso.