Signal boosters take your outside cell signals, amplify them if weak, and rebroadcast them inside. Bi-directional, they also help send a stronger signal back to your cell tower. As a result, you're able to stay connected in places where you couldn't before. Without existing cell signal, these devices will not work.
They are composed of an outdoor antenna, an amplifier, an indoor antenna, and coaxial cables. The strength of the booster and total coverage area vary on how powerful these individual components are.