The previous answer gives the basic details. Odysseus’s men show their greed on Ismarus. They take spoils, divide the women, and want more. Odysseus urges his men to leave immediately, but they, of course, do not listen. The end is the loss of life (deserved on account of their avarice).
Arguably, the most important point in the narrative is the mercy that Odysseus shows the priest of Apollo and his family. Odysseus spares Maron: the priest of Apollo and son of Euanthes. This act of mercy is rewarded. Maron gives Odysseus a goastskin bottle of black wine. This wine is very strong; it requires a dilution of twenty parts of water. In essence, the wine can be seen as twenty times stronger than ordinary wine. This detail might not seem important, but Odysseus uses this wine to make Polyphemus drunk. Without it, Polyphemus would have had Odysseus for dessert.
With me I had a goat-skin of the dark, sweet wine, which Maro, son of Euanthes, had given me, the priest of Apollo, the god who used to watch over Ismarus.
Hence, the importance of Ismarus can be seen in the gift that Odysseus receives.