It's pretty self-explanatory: it means your Physique is currently at 75% or more of its maximum possible value. By default, the max of any core stat is 5 times your current level, though certain status effects, perks and equipment can change that. For example, if you're level 10 and don't have any of those status effects etc., your Max Physique is 50, and 75% of that is 37.5; as such, if you want to get below 75%, set Physique to 37 or lower.