Aslan's Table was, as it says, the table of Aslan himself, which was located near the end of the Narnia world.
It was a long table, which was to be found on Ramandu's Island. It was set there at Aslan's bidding, for travellers who had travelled so far.
It was known as the beginning of the end of the world, because although you could sail further, it was the beginning of the end.
It was covered with delicious food and drink, which the Sun-birds came and ate every morning. The food was then magically replenished before the evening.White Witch's Stone Knife, which the White Witch had used to kill Aslan during the Winter Revolution, on the Stone Table.
It had been placed there centuries ago, presumably by Aslan, and was considered a holy relic.
No one was allowed to touch it.
The remaining three of the Seven lost Lords (Mavramorn, Revilian and Argoz) (great Telmarine lords, who had been on a voyage of explaration) had arrived at Ramandu's Island and had sat down to eat and drink, when they fell into an argument about what they should do next.
One of them wanted to carry on sailing East, and so continue exploring, another wanted to stay on Ramandu's Island and eat the food every day, and the other wanted to set sail for Narnia and go back the way they had come.
They found out that the only way to break the enchantment was to sail to the end of the world, and then return, leaving one person to sail on, and never return to the world. Caspian's crew did this, leaving Reepicheep to go to the end of the world.
When the crew of the Dawn Treader arrived back at Ramandu's Island they found the three lords awake.