It is one of the 3 Movable jewels. 

A trestle board is a framework consisting of (usually 3) vertical, slanted supports (or legs) with one or more horizontal crosspieces on which to hang or display an item.  Today, it is better known as an "easel". 

Some jurisdictions around the world call it a tracing board.  It would be somewhat of a "circular logic" task to argue the difference, as, while neither can be fully proven (in historical writings), the "Tracing board" may very well have predicated (come before) the use of the word "Trestleboard"  because lodges in Europe (which pre-date American lodges), use the word "Tracing Board".

Hiram's Tracing Board: Hiram Abif's tracing board is believed to have been made of wood, covered with a coating of wax.  Each day he would draw his Master architect's measurements and symbols into the wax in order to instruct his Master Masons of the work that was to be accomplished. 

At the end of the day, he would simply scrape off the wax and pour a new layer of hot wax onto the board to ready it for the next day's work. 

Masonic Tracing Board:   Much later, in the days where lodge was held in secret areas and on hills and vales, (valleys) once lodge was in session, the Tiler (or Tyler) would draw an oblong (rectangular) or oblong square depiction (image) into the dirt that represented the form of the lodge. 

Again, onto that tracing board was drawn the architect's plan...the working tools in the degree that was to be worked.

Masonic Trestle Board: Through the years, the Masonic Tracing Board progressed to charcoal or chalk on the floor of taverns where lodges were held back in the 1700s.  After the lecture, the Stewards or the Entered Apprentice, as a lesson in secrecy, would get a mop and bucket and remove all trace of these drawings. 

This, obviously, was a somewhat tedious and messy procedure, so cloths or rugs were created which could be laid onto the floor and simply folded up when the lecture was complete. 

Later, these cloths (or rugs) were placed onto a table.  As time passed, they were finally hung onto an easel...(a trestle board) much like a drawing board at a construction site where each workman could receive clear instruction as to what his specific participation entailed.

When the team's work was completed, it was obvious that each Master Mason not only understood their specific part in the undertaking, but how their part (no matter how small), contributed to the construction of the entire edifice (building). 

The meaning of the words "Nothing further remains to be done, according to ancient custom, except to disarrange our emblems" is a reference to the now antiquated use of these trestleboards (or tracing boards) during which the dirt on the ground was erased or the chalk marks on the floor of these lodges was mopped or scrubbed, to leave no trace of the form of the Lodge or the contents drawn thereon.

The reason why our lines of travel are at right angles within the lodge and thus the reason that we "square" the lodge is a "throwback" to the antiquity of the ritual. 

If the brethren were to walk atop the markings made in the dirt on hill and vale; atop the chalk on the floor of the taverns; or tread upon and thus soil the cloths or rugs used to provide the workings of that degree, the message of that lecture which was being worked could be partially or fully destroyed.

Therefore, "Squaring the Lodge" in a semi-military-like precision, goes back many centuries as the means of preserving the ritual and the degrees being worked so as not to destroy the symbolism of their markings before their usefulness on that day has been completed. 

...and now you know "The Rest of the Story!"

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Many modern-day lodges now simply use a bulletin or electronic newsletter to notify the brethren of a degree which will be worked.  Others use PowerPoint or slide presentations.