You walk into the room, and see that the wall is red. How do you know this? Well, on the most basic level, a light source is radiating in the visible wavelengths, and that wall does a fair job of absorbing all of them aside from a few in the red section of the spectrum, which is reflected back. This reflection of light is focused by your eye, where it impacts your retina, exciting nerves, and causing a distinct mental state. That's the science of it. But that isn't how you know it. What you have is an experience. You are experiencing a red wall. This experience involves other experiences you have had. Your experiences assemble a context for this red wall.
Now, I've played a nasty trick on you. You're actually delusional. The wall is really blue. The fact differs from your experience. Does this do anything to diminish your experience? You still have had the experience of a red wall, even if it is blue.
So this starts us pouting off to a Cartesian doubt, but there's really no need to pout off in that direction in a huff. Without a method of correlating our experiences with others, we have no other tool for understanding our world. When we start examining certain tools like language, society, communication networks, etc. we start seeing how experiences can be shared. In fact, right now, I share with you my experience of this philosophical thought. In turn, you process this as an experience.
I'm bored, or there'd be more of this.