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.
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?