Short answers, pulled from the story.
Lagertha was a legendary Viking shieldmaiden and ruler from what is now Norway, recorded in the ninth book of Saxo Grammaticus's twelfth-century work the Gesta Danorum. Saxo describes her as the onetime wife of Ragnar Lodbrok and as a key fighter in his military campaigns. Her name appears in Saxo's text as Lathgertha, though it is also recorded in variant forms including Lagertha, Ladgertha, and Ladgerda.
Most historians consider Lagertha legendary rather than historical. Judith Jesch describes the first nine books of Saxo's Gesta Danorum, which include the Lagertha tale, as "generally considered to be largely fictional." The scholar Nora K. Chadwick considered it very probable that Lagertha is identical with Thorgerd Holgabrud, a Norse goddess worshipped by the Norwegian ruler Haakon Sigurdsson around 937-995.
Lagertha arrived with 120 ships when Ragnar sent to Norway for help during a civil war. At a critical moment in the battle, when Ragnar's son Siward was wounded and the soldiers were wavering, she led a charge around the back of the enemy line and reversed the panic. Saxo describes her as having "a matchless spirit though a delicate frame."
Historians Hilda Ellis Davidson and Nora K. Chadwick identified strong parallels between Lagertha and the goddess Thorgerd Holgabrud. Thorgerd was worshipped by Haakon Sigurdsson, who lived at Hladir (Lade), and the Gaular valley where Saxo places Lagertha's home was the documented center of Thorgerd's cult. Lagertha's likely Old Norse name, Hladgerd, derives from Lade, and the Flateyjarbók describes Thorgerd assisting Haakon in battle in a way that closely mirrors Saxo's description of Lagertha.
The first ballet to feature a Nordic theme was Lagertha, created by choreographer Vincenzo Galeotti in 1801. It was set to music by Claus Schall and performed at the Royal Theater, where it was a significant success. Galeotti based the ballet on Christen Pram's 1789 historical drama of the same name, which in turn drew from Saxo Grammaticus's twelfth-century account.
In the 2013 TV series Vikings, Lagertha is played by Katheryn Winnick and appears as a main character. The series portrays her as a shieldmaiden and as Ragnar's first wife who later rules as a jarl and then as a queen in her own right. The portrayal is broadly based on Saxo Grammaticus's account.