A tidal force is simply the force produced by gravity which acts differently on different parts of an extended body. For example, near the Earth, gravity is a 'radial' force pointed inward to the center of the Earth, and which decreases as the inverse-square of the distance to the center. This is not a uniform force field, because for a large object like the Moon, the force of gravity on the front side closest to the Earth, is slightly stronger than on the more distant back side of the Moon. This results in a net force which is felt on the Moon, and which tries to deform the Moon into a football shape, front to back. Also, the direction of the center of the Earth as seen from the 'west' edge of the Moon, is not the same as the direction from the 'east' edge of the Moon, so there is a net deforming force acting from side-to-side due to the shape of the Earth's gravitational field. These two components are the 'tidal field' of the Earth produced entirely by gravity acting slightly differently at neighboring locations in space near the Earth. At a sufficiently great distance, these 'differential' forces become vanishing small and the gravitational field begins to look more and more uniform.