A field is something that has a magnitude and a direction at every point in space. Gravity is a good example - we know there is an acceleration due to gravity of about 9.8 m/s2 down at every point in the room. Another way of saying this is that the magnitude of the Earth's gravitational field is 9.8 m/s2 down at all points in this room.
Gravitational field: g = F/m
where F is the force of gravity.
We can draw a field-line pattern to reflect that, near the Earth's surface, the field is uniform. The strength of a field is reflected by the density of field lines - a uniform field has equally-spaced field lines.
If we zoom out and view the Earth from far away, we get a non-uniform pattern. In fact, the pattern is radial - the lines are further apart as they get further from the Earth, reflecting the fact that g decreases with distance. At every point, though, the field-line pattern shows the direction of the gravitational force that would be experienced by a mass placed at that point.