What are magnetic field lines?
In Magnetic Effects of Electric Current Class 10 notes The field lines, or field lines of a magnet, are imaginary lines that show the magnetic field of a certain magnet. When we let iron fillings settle around bar magnets, they automatically form a pattern that resembles the magnet's field lines. A compass can also be used to determine the field lines of magnets. It's a vector quantity, which means that it has both magnitude and direction.