It's a reference to Undertale
However in undertale it's an ability only two characters have
The player
And flowey
Basically, determination let's them come back from the dead by rewinding time
In bossfights, determination entities are a classification of creature that cannot be killed, most don't know why but anyone with a dimensional diamond can see the truth, they are just as mortal as everything else. But they can see attacks coming because it isn't the first time they were attached, or killed
Trial and error, that's why we can't kill them
However there is one way to beat them, by making them rage quit. Meaning they willingly forfeit. However, this is a rare weakness