Like heroes, Legendary Monsters have advantaged (+) or disadvantaged (–) saves. For example STR++, WIL– means that this monster would roll STR saves with advantage 2 (two additional dice), and WIL saves with disadvantage.
Saves (Legendary)
GMG v1 p. 42
Related Rules
4Bloodied (Legendary)Monsters / Legendary Monsters
Optional ActionsMonsters / Legendary Monsters
Running Legendary MonstersMonsters / Legendary Monsters
SavesFundamentals / Stats, Skills & Checks