"They have potatoes and bread."
Translation:Ils ont des pommes de terre et du pain.
Quite often the second part of a compound noun is not pluralised, but the first. This also applies in English.
For example, in Australia, we have a representative of the Queen known as the "Governor-general". If we speak of this post in the plural, they are "Governors-general".
Eg "The first three Governors-general of Australia were John Hope, Hallam Tennyson and Henry Northcote".
It is an error that is commonly made in English. Other examples are "brothers-in-law" and "sisters-in law".
Are you familiar with indefinite articles? Un and une ?
They have a potato → Ils ont une pomme de terre.
Now, the plural of the indefinite article (un or une) is des.
So, think of it in English terms. "I have a potato" / "I have potatoes".
As a rule, treat every French noun as needing an article. If the definite article (le, la l' or les) hasn't been used, then the missing article is the indefinite article.
As the English sentence states "They have potatoes ..." plural you know the missing article is des → the plural of une.
I hope that helps. Let me know if you need further explanation.
Bon courage !