Short answers, pulled from the story.
Serkland means "land of the Serkir", a term usually identified with the Saracens. It was the Old Norse name for Islamic lands to the east, originally referring to the region south of the Caspian Sea before expanding to cover all Islamic territories.
The etymology of Serkland is disputed. Proposed origins include a derivation from "Saracen"; from sericum, the Latin word for silk, suggesting a connection to the Silk Road; from the Khazar fortress of Sarkel; or from serkr, the Old Norse word for a shirt or gown, meaning "land of the gown-wearers".
The runestones that mention Serkland include Sö 131, Sö 179, Sö 279, Sö 281, the Tillinge Runestone, and the lost runestone U 439. Sö 179, raised around 1040 at Gripsholm Castle, is one of the Ingvar runestones and commemorates a Varangian loss during a raid into Serkland.
Several sagas mention Serkland: Ynglinga saga, Sörla saga sterka, Sörla þáttr, Saga Sigurðar Jórsalafara, Jökulsþáttur Búasonar, and Hjálmþés saga ok Ölvis. The term also appears in verse by the eleventh-century skald Þórgils Fiskimaðr and the twelfth-century skald Þórarinn Stuttfeldr.
Serkland originally referred to the land south of the Caspian Sea. Over time the term expanded to cover all Islamic lands, including parts of Africa and possibly even Muslim Sicily.
Gripsholm Castle is in Sweden. The runestone Sö 179, raised there around 1040, is one of the Ingvar runestones and commemorates Varangians who died during an ill-fated raid into Serkland, making it one of the most notable stone memorials connected to Norse expeditions into Islamic lands.