A Eucharist wafer, also known as a "Communion wafer", "a Eucharist" or even "the Eucharist" (as if there were only one of them), is a circular edible cracker used in the Catholic ceremony of the Eucharist.
Catholic churches typically purchase the crackers from mass producers at a few pennies each. Prior to the ceremony, the wafer is ritually blessed (consecrated) by a Catholic priest, at which point the wafer is referred to as a consecrated host. Each participating parishioner receives a "consecrated host" from the priest, and typically consumes it before returning to her/his seat -- although adherence to this expectation varies.
According to official Catholic doctrine, the wafer has at that point been somehow converted or transformed into the "Body of Christ". The doctrine also states that there is no way to detect this change (i.e. to tell whether or not a given cracker has been "transubstantiated"), as the transformation involves an apparently abstract concept which they misleadingly refer to as the "substance" of the wafer; the physical properties of the cracker are not altered. It is unclear why Catholics believe that a change has taken place, as the exact nature of the change is neither clear nor detectable.
The doctrine apparently leads to the conclusion that improper treatment of the cracker is somehow harmful to Jesus Christ – even when such improper treatment merely involves not eating the cracker, or even merely delaying such consumption until after returning to one's seat (see 2008 sacred wafer scandal).
Despite official doctrine, however, many Catholics believe (in accordance with common sense) that the transformation is meant to be purely metaphorical, and that improper handling of the wafer is a minor social crime (on the approximate order of littering or burping loudly in a quiet room), and regularly take the cracker back to their seats before eating it.