The answer depends on the denomination of the Christian Church the altar is in.
Typically there is nothing in an altar, it usually serves as a table to hold the items needed in liturgical rites in a Christian church. These items could include books (prayer book, or service book), chalice and patten for Communion, candles, and a cross or crucifix.
Some altars -- usually those in Episcopalian, Anglican, or Roman Catholic churches -- will have a compartment for the consecrated host (bread) and wine used in the rite of Communion.
Some Roman Catholic churches may have relics in the altar. At one time, and possibly at present, it was believe that certain relics would specially consecrate a worship place, and they would be placed in the altar (built to hold them).