system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How "Noise Reduction" headphones work

Like the Bose ones, or any others.
gpt
You are probably thinking of noise cancelling headphones. 

In these headphones noise is passively and actively blocked. Noise is passively blocked due to the headphones dense padding. Thicker padding = less ambient noise will be heard. 

Active noise cancellation is a lot more interesting. You know sound travels in waves right? Sound waves have crests (high points) and troughs (low points). The headphones detect the formation of these ambient waves (the outside sound that you hear) and cancel them out by projecting an opposite wave. You might be familiar with basic trigonometry. Think about what sin and cosine waves look like on a graph. The noise cancellation is producing sound waves that nullify the ambient waves. 

When hearing outside noise, at every crest your headphones will produce a trough and at every trough your headphones will produce a crest.