Rich and poor are adjectives.

‘a rich and famous family’

With the definite article, they become phrasal nouns: "the rich" and "the poor".

‘every day the split between the rich and the poor widens’

Rich and poor are adjectives.

‘a rich and famous family’

With the definite article, they become phrasal nouns: "the rich" and "the poor".

‘every day the split between the rich and the poor widens’

That is, those who are rich and those who are poor.