Carbon-14 (C14) is a type of carbon that is found in the air, food, and also in our bodies. It's also found in things like wood, rocks, and seashells. Every living thing has a certain amount of Carbon-14 in it, and when they die they stop taking in new Carbon-14.
The 774-775 Carbon-14 spike is a time period when there was a sudden, unusual amount of Carbon-14 in the air and the things that live on our planet. Scientists think that this was caused by a huge volcanic eruption that happened around 775 A.D. shooting lots of Carbon-14 into the air. All the things, like trees and plants, that were alive during that time, now have this spike of Carbon-14 in them.
Using Carbon-14, scientists can use this information to look back in time and study things, like what the climate was like, or how the climate has changed over time. Carbon-14 is like a time machine that lets us look at the past.