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what is really meant by magnetic field

A magnet produces a vector field, the magnetic field, at all points in the space around it.  It can be defined by measuring the force the field exerts on a moving charged particle, such as an electron.  The force (F) is equal to the charge (q) times the speed of the particle times the magnitude of the field (B), or F = q*v x B, where the direction of F is at right angles to both v and B as a result of the cross product.  This defines the magnetic field's strength and direction at any point. 

What creates the magnetic field?

A magnetic field can be created with moving charges, such as a current-carrying wire.  A magnetic field can also be created by the spin magnetic dipole moment, and by the orbital magnetic dipole moment of an electron within an atom.