As Blake stated, the Tracing boards or Masters Carpet were charts per say that illustrated to the candidate the emblems and symbolism of Freemasonry. In the early days the Tyler would draw on the floor the Carpet pertaining to what degree the Lodge may be opening on. It is said that it would be the candidate that would wash the floor to removed the drawings.
Oil paintings came into play and artist would paint these Master Carpets for the Lodges. They would be hinged on frames or would roll up so they would afterwards be stored and not seen. These Master Carpets would have most of the important emblems of Freemasonry upon them. The steps, columns, pavement, some had an arch, hourglass, etc etc. Later charts were printed around the 1840's and used by Lodges. These charts would have all three degrees on one, or one degree on a chart and then there would be three of them. Like the Master Carpets, they too would be mounted in a way that they could be rolled up for storage or in some cases on rollers that were mounted in boxes. When the degree was over, the chart could be rolled up like a shade into the box and no one could see it.
These Carpets or Trestle-boards were first used in England, and were painted on boards, in early Freemasonry, one degree per board. Like Blake mentioned, early Freemasonry met in Taverns, so all Masonic regalia had to be stored and placed out of sight from Non-Masons. Later glass slides were produced and sold by regalia dealers for the use of degree work in the Lodges.
As Blake mentioned it was the early type of power point. We still have our 200 year old Masters Carpet hanging in the East of our Lodge room and at times use it still for degree work. Other Lodges still use some of the older charts to explain and show at the same time emblems to the candidate. Some English regalia dealers still sell a similar version of these Carpets and Boards today.