To 'measure' means to form some concept of something (at the label level).
We seem to get the same results over and over again. Why? One explanation is that WIGO exists and that the object level is an actual reflexion of WIGO. This explanation can be called 'realism'.
Which of the two slits a given particle goes through in the double-slit experiment is random. So can we measure it with detectors to find out which slit which particle is going through? Nope, we can't, because putting the detectors there destroys the interference pattern. If you put a detector at just one of the two slits, that would allow you to measure it. Obviously doing this destroys the interference. This is weird because putting the detector at slit 1 seems to be affecting what particles at slit 2 are doing. Therefore, some variable must be distributed across both slits (because the influence is distributed)