That's a technical expression meaning it can't be on a collision course with an asteroid or comet or another planet. . . . Next, the planet has to be cool, which means it can't be too close to the star, so its crust can solidify. Any planet forming near enough to a star to avoid becoming frozen is also too near to contain carbon, nitrogen, or water. These chemicals seem to be necessary for life, and all of them would have been vaporized by a young, hot star. With any luck, comets carry them toward the new planet, which is apparently how earth got seeded with the raw material of life.
Life also needs an atmosphere.
Water vapor, carbon dioxide, and other gases trapped in rock can form one, but the planet needs to be heavy enough that its gravity can hold on to it. Earth Is Uniquely Habitable. By some curious coincidence our earth has the precise combination of conditions essential for a planet to be habitable. Christian astronomer Hugh Ross lists 33 different constants in our solar system that make the earth capable of supporting human life. Apparently other planets rotate around their stars in egg-shaped orbits that make temperature variations so extreme that life as we know it could not exist on them.