sih yaht laih tōng
是日 is this day, and 例湯 is regular soup and by itself can also mean soup of the day, so 是日例湯 is today's soup of the day.
Soup of the day is a standard feature of (Hong Kong cafes), often included in a set meal.
soup
hot and sour soup
egg drop soup
oxtail soup
West Lake beef soup