Readers learn that Mrs. Wormwood is so exhausted from her bingo afternoons that she is not able to make home-cooked dinners. Therefore, the family always eats either frozen dinners or fish and chips that she picks up on the way home. The family always eats in front of the TV, watching whatever show is on.
This is all part of the indifference with which the Wormwood parents treat their children, especially Matilda. Her intelligence puts them to shame, even when she is only five years old, so they do their best to hold her down.
The lack of nutritious food is a symbol of the lack of emotional depth, intellectual stimulation, or moral values in the Wormwood family. The parents are concerned with making money, even if they have to cheat people to do so, and merely want to go through the motions of family life in the easiest way possible.
It is no wonder that rage builds up in Matilda around these family dinners. She already knows there is more to life than eating bad food while watching silly TV shows. At a young age, she realizes she would rather be up in her room reading Dickens or other good literature, just as she realizes she wishes she were in a better family.