"Broad grass", or "broad-leaf grass" is a term used for many types of grass that have wider leaves than most other grasses. In the franchise however, it is a specific species of grass that has properties that cause itchiness when rubbed against the skin. It's suggested it also has a scent that can attract sheep even when it is not visible.
In the episode "The Flight Stuff", Snotlout thinks he's dying and is training Gustav to take his place as a Dragon Rider. The twins enjoy pranking him into thinking he keeps witnessing the five "Signposts of Valhalla" to support his belief that he's about to die. They tell him that putting broad grass in his underwear can prevent saddle chafing, when in reality they know sheep will be attracted to him, and cause him to believe he's witnessing the "Shepherd's Curse"; one of the Signposts. Along with the sheep crowding him, the grass also causes Snotlout's backside to itch uncontrollably.