The quantum world seems alien and far away, yet the particles around us can behave in a quantum mechanical way. But how can we - living in the 'classical' world - still notice anything going on in this strange quantum world? We can do this by measuring.
Do you appreciate the difference between quantum and classical measurements?
There are many people who came up with a theory how a measurement collapses the quantum wavefunction. Check out the Wikipedia page for an extensive overview: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics.
What is your favourite interpretation?