For most of Christian history a fixed altar was required to contain a relic, sealed into a small cavity in the altar stone itself, a custom traced to the early practice of celebrating the Eucharist over the tombs of martyrs.
Construction and consecration
A stone altar has a flat mensa, the table surface, resting on a support of stone, brick or wood, and a bishop consecrates it by anointing it with chrism at the four corners and the center, a rite that only a bishop, not a parish priest, may perform. Modern liturgical reform relaxed the relic requirement and revived the freestanding table altar facing the congregation, a return to the simpler form used before the post Constantine shift toward the fixed stone altar built over a martyr's tomb or a portable relic.
Who may use it, and how the form has moved
Only an altar that has been properly consecrated or at least blessed may be used for Mass, and moving or altering a consecrated altar is not undertaken casually, since the consecration is understood to belong to that specific stone rather than to the building around it. Portable field altars, small enough to fit in a case, exist for military chaplains and missionaries, carrying a single small consecrated stone rather than a full altar slab, proof that the requirement travels with the smallest sufficient object rather than the largest.