Imagine that frequency is color (actually true). Tuning is searching for that color. So you, the receiver, are tasked with finding a (for example) blue light and counting the blinks. Easy until you surround that blinking blue light with 1000 more just like it all blinking at different speeds. Complete the metaphor with the fact that directionality is hard or often impractical so imagine your field of focus is slightly blurred, like looking through the viewfinder of a camera that isn't focused. All that other blue light is interference.

There are only 11 or so distinct "colors" in 802.11g and some of them are close to each other and quite hard to tell apart.