For a planet, the habitable zone is the distance from a star that allows liquid water to persist on its surface – as long as that planet has a suitable atmosphere.
In our solar system, Earth sits comfortably inside the Sun’s habitable zone. Broiling planet Venus is within the inner edge, while refrigerated Mars is near the outer boundary.
For larger, hotter stars, the zone is farther away; for smaller, cooler stars, it can be very close indeed. Finding these “just right” planets in the habitable zone is one of the keys to finding signs of life.