Du is the 2nd-person singular subject pronoun ("you" referring to one person), but ni is the 2nd-person plural subject pronoun ("you" referring to multiple people). Both happen to translate as "you" in English.
"De är äter bröd" would be like saying "They are eat bread" in English.
We just don't have the present continuous construction are eating that English has. Our de äter covers both they eat and they are eating.