A double-entendre is a figure-of-speech used in literature where a word or phrase is meant to suggest one of two meanings. Very often, as in the example above, double entendres are sexual in nature.
It has oft been suggested that Beowulf was a mighty man of valor, both physically and sexually. The scene of his fight with Grendel's mother is riddled with potentially sexual double entendres, and certainly the most recent film depiction of the epic depicts this scene as such. From the moment he first enters the "heaving water," this scene is suggestive of more than just battle.
They wrestled; she ripped and tore and clawed at him, bit holes in his helmet, and that, too, failed him; (1526-27)
At one point in the fight, Grendel's mother gets Beowulf "on his back" and the language suggests she is straddling him:
Squatting with her weight on his stomach, she drew a dagger, brown with dried blood, and prepared to avenge her only son. (1545-47)
Again, suggestive of more than just battle, such double entendres bring a new sense of depth to the poem. When looked at through this lens, Beowulf may be successful in battle but shown as overpowered sexually.