The amount of information coming into your brain through your senses at a time is huge, so you can’t process all of it. Your attention is focused mostly on one thing at a time. This thing might be a particular spatial location (e.g., your web browser window), a particular person’s voice (e.g., the person you’re talking to at a loud restaurant), a particular visual property (e.g., looking through your sock drawer for a black sock), etc.

It’s quite hard to focus on more than one or two things at once. Watch this video to see how hard it is to keep track of four moving objects at the same time.

People are pretty good at paying attention to one thing while ignoring other things. The cocktail party effect refers to the fact that even with many different sources of sounds around you, you can listen to one person talking and “filter out” the rest.

Here are two videos demonstrating how, when paying close attention to one thing, you might not notice other things:

Here is a video in which people don’t even notice when a person is replaced with a different person.

You don’t necessarily have to be paying close attention to something else to not notice something. This example shows how you might not notice something gradually changing if you’re not specifically paying attention to it.

Sometimes, you don’t notice things because your mind is busy processing something you’ve just perceived. When you see things quickly one after another, there is an attentional blink shortly after each thing in which you often won’t notice the next thing. You’re also less likely to notice something if you’ve just seen the same or a similar thing - for example, if you’re reading and the same word occurs twice in the same sentence; this is called repetition blindness.

It’s possible to perceive something even if you don’t consciously notice it; this is called subliminal perception. Subliminal messages probably don’t influence your decisions as much as the movies might have you think, but they really can influence your judgments - for example, if you are subliminally shown a positive or negative picture, and then shown a picture of a person, you will likely have a more positive or negative attitude toward the person depending on the subliminal picture.

Here are a few websites with some more cool demos: