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Field lines are imaginary lines showing the direction of vectors in a (mathematical) vector field (as well as any physical field the mathematical field represents), often included in illustrations of fields. In an illustration, labeling might reveal which of the two possible directions the vectors are pointed, since simple lines don't reveal that. Field lines are commonly used to illustrate magnetic fields (magnetic field lines) and often the term is used to describe phenomena that follow paths that could form a field line, e.g., "along the field lines, ...". A simple experiment of a magnet under a piece of paper with (tiny stick-like) iron filings sitting on top results in the filings illustrating the field in a "field line" manner. Field lines can also be used to illustrate other mathematical and physical fields, such as electric fields and gravitational fields as well as fluid flow and heat flow.
Field lines follow the direction of a gradient of a related mathematical field representing potential (e.g., the gravitational potential of a gravitational field). The field lines are perpendicular to lines that show equal amounts of potential in the manner of the altitude lines of a topographical map or the isobars of a weather map. (Such lines illustrating a physical field are termed equipotential lines.) A map of topography using lines analogous to field lines would have lines following the ground's steepest direction rather than lines in the direction along which ground is at equal altitude.