The River Arno flows through Florence.
This Plaque was located on a building in Piazza di Santa Maria Novella.
Longfellow, best known to me for "Paul Revere's Ride" translated Dante's Divine Comedy into English. According to www.bootlegbooks.com:
In 1861, the happiness and serenity of Longfellow's life were suddenly broken by the shocking accident which caused the death of his wife. Sitting in the library of their home, sealing some packages of their little daughter's curls, Mrs. Longfellow's dress caught fire. She died the following day. To occupy his mind and alleviate his sorrow, the poet began a translation of Dante. Upon this he worked at intervals for several years. The Divine Comedy was completed in 1867; it holds a place among the best versions of Dante's work in English.
Dante lived in Florence, but was expelled and is buried elsewhere. Longfellow visited Florence after completing his translation. I can't read Italian so I'm not sure what this plaque says, but I assume it refers to this history.
Ice cream stores are common in Italy, and the ice cream usually looks (and is) delicious.