It means Letters of Administration. Generally, when a person dies without a will, a court will appoint someone to handle their estate. The court issues Letters of Administration to show that the administrator been appointed to handle the estate.

If a person dies with a will, a court will generally appoint someone to handle the estate in accordance with the will. This is generally called a Grant of Probate, but in some areas this might also be called Letters of Administration with Will Attached, or something similar.

Letters of Administration are often granted to family members or close friends (or maybe a lawyer on their behalf). Before a court will grant them, they usually require the administrator to advertise. This allows anyone else who might have a claim to challenge the appointment. There are a lot of these ads in your link, along with people selling the property of the deceased.

http://www.usgennet.org/usa/ga/topic/news/journal1808.htm

This is an example of the website.

Tip: Use Ctrl+F to search for "admr"