The meaning of the whole poem is: Red beans grow in the south, and spring comes and grows luxuriantly. I hope you can pick more red beans, because it is a symbol of acacia.
Wang Wei did not put too much pen and ink into the specific description of red beans. He started from caring about the growth of red beans, to persuading the other side to collect more, and finally returned to the main idea and pointed out the significance of red beans.
This is a poem about things, but the poet's original intention is to convey his feelings through things and express his love for southerners. The whole poem contains deep love for things.