Okay, so imagine you have two ears - one on each side of your head. When you hear something, your brain uses the information from both ears to figure out where the sound is coming from.
A Blumlein pair is kind of like that, but for recording sound. Instead of ears, it uses two microphones that are placed close to each other, facing in different directions. When sound waves reach both microphones at the same time, they create a really cool effect - it makes you feel like you're right in the middle of the sound, like you're there with the musicians when they were playing!
This Blumlein pair technique was invented by a man named Alan Blumlein a long time ago, and it's still used by audio engineers today to make recordings that sound really natural and lifelike. Pretty cool, huh?