means Stuff that's worthless, broken, or no longer wanted — the cluttered category for things too useless to keep but too vaguely promising to throw away.

from The oldest 'junk' was nautical: in Middle English a 'junk' meant a length of old rope or worn-out cable too frayed to trust at sea. Sailors saved this discarded cordage and picked it apart for oakum to caulk seams, so 'junk' came to mean cheap leftover odds and ends — and from there spread to all manner of trash and worthless gear. (Note the separate 'junk' meaning the Asian sailing ship is a different word entirely, borrowed via Portuguese from a Malay or Javanese term, so don't let the two ropes tangle.)