While in-ear headphones can create head-thumping bass by sealing your ears off from the world, they also isolate you from the sound of cars, bikes, and other people, which can be dangerous.
While most headphones transmit music through the air, entering your ear canal and vibrating your eardrum, bone conduction headphones are different. They sit outside your ear, vibrating the bones of your head until the sound reaches the cochlea—the same place the eardrum sends sounds to be converted for your brain. That means your ear canal is free to listen to the sounds around you.
This technology has been around for a lot of years but I’ve never used them. Many people who hate sticking earbuds into their ears may benefit from them though.