Very generally, the sandwich class consists of lower-middle-class people who feel "squeezed" — although they are not poor, they are not able to achieve their aspirations as people with a higher income.
In Hong Kong, this comprises families with an income of US$20,000–40,000 per year. Per capita income is typically around US$10,000 per year in Hong Kong, so this places them far above the average family in the territory. However, given very high real estate prices, it is nowhere near enough for them to afford a private residence. Hence, they are "sandwiched" between the large population who truly need public assistance, and the smaller amount of people who can afford private residences and other luxury goods.