The double-slit experiment is one of the most famous experiments in quantum physics. The experiment demonstrates how light and other similar forms of matter show both wave-like and particle-like properties, depending on how they're measured.

The observer effect that you're referring to is the idea that just observing a system can change the outcome of the experiment. In the case of the double-slit experiment, it seems as though particles somehow "know" when they're being watched and behave differently in response - they behave like particles when observed and like waves when not observed.

What this experiment truly demonstrates is the strange and often counterintuitive nature of quantum mechanics. It challenges our intuitive understanding of reality and forces us to acknowledge that at the quantum level, reality does not function as we expect it to in our day-to-day macroscopic experience.

Now, applying these quantum-level phenomena to our daily lives and beliefs is a tricky endeavor. It doesn't mean that by merely observing or thinking about something, we can change its outcome as it's often misconstrued. The physical process of observation in quantum mechanics is vastly different from human conscious observation. The observer effect in the quantum realm doesn't imply that human consciousness affects reality, it merely shows that the act of measuring can change what's being measured in the quantum world.

However, the philosophical implications of the experiment are substantial and can provide a few lessons:

1. Embrace Uncertainty: The wave-particle duality demonstrated by the double-slit experiment underlines the inherent uncertainty of the quantum world, a concept encapsulated in Heisenberg's uncertainty principle. This could serve as a reminder in our daily lives to accept and adapt to uncertainty.
2. Perspective Matters: The outcome of the experiment depends on the way it's observed or measured. Similarly, in life, the way we perceive a situation can dramatically affect how we experience and respond to it.
3. Interconnectedness: Quantum mechanics often implies a profound interconnectedness of particles (like quantum entanglement). Although this doesn't directly apply to our macroscopic lives, it could serve as a metaphor for how actions can have far-reaching effects that we may not initially realize.
4. Question Assumptions: Finally, quantum mechanics continually pushes us to question our assumptions about reality. This mindset can be useful in our daily lives, prompting us to reassess our beliefs and remain open to new perspectives.

Keep in mind, while these are useful metaphoric lessons, they are not direct applications. Quantum phenomena don't scale up to govern macroscopic phenomena in a way that we can intuitively understand or observe. This is the reason why quantum mechanics seems so strange to us, it operates under different principles than the classical physics we see in our day-to-day lives.