Help me understand "Subject to" Existing Mortgage

[Louise Whidby]

I probably won't ask this question where it's understandable, because I am confused about what I was reading. Wholesaler finds Motivated seller that has an existing mortgage of 61k. Wholesaler tells seller that he will give him 3k cash at closing, and negotiated leaving the existing loan in place for 1 yr..... Could someone please explain what leaving the existing loan in place actually means and who does it benefit? How does it work?