To add to the good answers already posted, think of how you "observe" something in day-to-day life. When you see an object, you see it because a photon (or many photons) from some light source bounced off of it and went into your eye. In the quantum world, this bounce itself changes the object you're looking at. The electromagnetic interaction between the mystery particle and the photon puts the particle in a different state than it was before. This is generally what we mean when we say that observing a quantum state "changes the outcome".
There is definitely some additional nuance as other posters have remarked, and observation is not always as simple as bouncing a photon. The delayed choice quantum eraser is a fun experiment that shows you can actually wait to "choose" whether or not to "observe" a quantum event until after it happens! Quantum is weird.
ass_bongos t1_iriunq0 wrote
Reply to What counts as "observation" in quantumn theory? by Iron_Rod_Stewart
To add to the good answers already posted, think of how you "observe" something in day-to-day life. When you see an object, you see it because a photon (or many photons) from some light source bounced off of it and went into your eye. In the quantum world, this bounce itself changes the object you're looking at. The electromagnetic interaction between the mystery particle and the photon puts the particle in a different state than it was before. This is generally what we mean when we say that observing a quantum state "changes the outcome".
There is definitely some additional nuance as other posters have remarked, and observation is not always as simple as bouncing a photon. The delayed choice quantum eraser is a fun experiment that shows you can actually wait to "choose" whether or not to "observe" a quantum event until after it happens! Quantum is weird.