So, a heartbeat star is like a big ball of gas in space that keeps getting bigger and smaller over and over again, like a beating heart.
Picture a big balloon that someone blows up and then lets the air out of, and then blows up again, and then lets the air out of again - that's kind of what a heartbeat star does.
Scientists call this process "pulsation." It happens because the star is heated up at its center, which causes the gas to expand and get bigger. But then, the gravity of the star pulls the gas back in, which makes it get smaller again.
This expanding and contracting is what gives the heartbeat star its name, because it looks like it's "beating" in the sky.
Heartbeat stars are really important for scientists to study because they can give us clues about how stars are formed and how they change over time. Plus, they're pretty cool to look at!