Every object that has mass has a gravitational field around it. Any other mass in that field feels a force of attraction towards the object. Gravity is a non-contact force, so the objects do not have to touch. Gravity always attracts; it never pushes masses apart.
The strength of the field depends on the mass of the object producing it. A book or a person has a tiny field that you can never notice. A massive object such as a planet or a star has a much greater field strength, so it pulls strongly on things nearby. This is why the Earth pulls you down, but you do not feel your desk pulling you sideways.
Gravitational field strength has the symbol g and is measured in newtons per kilogram (N/kg). At the Earth's surface g is about 10 N/kg. Other bodies have different values: about 1.6 N/kg on the Moon, where the field is weaker because the Moon has less mass, and about 25 N/kg on Jupiter, where the field is stronger because Jupiter is far more massive.
