Magnetic field lines are the imaginary lines which are nothing else than the pictorial representation of the field.
A magnet has always two poles. Mono pole magnet does not exist in nature.The two poles of a magnet are known as south pole and north ole.
The magnetic filed lines originate from the north pole of a magnet. Unlike electric field lines, magnetic field lines form close loop.The direction of magnetic filed lines are from north pole to south pole outside of the magnet. The direction will be reversed inside the magnet i.e it will be directed from south to north.
Hence, the direction of the magnetic field lines are from south pole to north pole inside any magnet.