"We do not eat expensive food."
Translation:हम महँगा खाना नहीं खाते।
है (along with its other forms like हैं, हूं) is the present tense form of the verb 'to be' and is used to indicate that a sentence is in the present tense.
When you have a negative sentence (containing नहीं) the use of है is optional.
The only exception is when the sentence does not consist of any other verb (eg: हम बुरे नहीं हैं - 'we are not bad' should compulsorily have a हैं)