Magnetic field lines depict the strength of the magnetic field, dense lines in a region imply a strong field.
Lines are drawn as per our convenience. We needn't draw infinite lines but just relative amounts: where the field is stronger, more lines; where it isn't, less lines.
What I wanted to know was what do these lines imply when they are formed in the magnet and iron filings experiment. All this while we find that these lines can be infinite with relative spacings to depict field strength. It seems that these lines are imaginary and only for our convenience. Yet iron filings form them. So what do they imply?
Why do the iron filings follow only specific lines around the magnet? What about the spaces in between two adjacent lines? What are those spaces supposed to be? Why wouldn't the iron filings fill those up as well? Are those spaces supposed to imply absence in field?