Q. 1. What is an antimension and what is it used for?

A. 1. The Antimension is a consecrated corporal of a kind used only in the Greek Rite. It is called in Russian and Slavonic antimins, and answers substantially to the portable altar of the Roman Rite.

It consists of a strip of fine linen or silk, usually ten inches wide and about thirteen to fourteen inches long, ornamented with the instruments of the Passion, or with a representation of Our Lord in the Sepulchre; it also contains relics of the saints which are sewn into it, and certified by the bishop.

It is required to be placed on the altar in Greek churches just as an altar-stone is required in the Latin churches, and no Mass may be said upon an altar of that rite which has no antimensium. It is unfolded at the Offertory quite like the Latin corporal. Outside of the Mass it rests on the altar folded in four parts, and enclosed in another piece of linen known as the heileton.

Originally it was intended for missionaries and priests traveling in places where there was no consecrated altar, or where there was no bishop available to consecrate an altar. The bishop consecrated the antimensium almost as he would an altar, and the priest carried it with him on his journey, and spread it over any temporary altar to celebrate Mass. Originally, therefore, it stood literally for its name; it was used instead of the Holy Table for the Sacrifice of the Mass.

[Source: http://www.newadvent.org/cathen/01563a.htm]