What does it mean to say that someone has been excommunicated from the Catholic Church?
To be excommunicated is to be literally separated from the communion of the Church. This separation implies both being spiritually outside the boundaries of the community of faith, and excluded from the sacraments of the Church, particularly the Eucharist. Generally speaking, Excommunication is a penalty incurred for specific public acts that the Church finds singularly offensive.
Sometimes, a person effectively excommunicates themselves, incurring an automatic, or latae sententiae excommunication. Examples of this include physically attacking the pope, directly violating the seal of confession if you’re a priest, or directly procuring an abortion. In many other cases, a serious penalty like excommunication should be imposed only after a trial before a church tribunal. Even within the Church, people are entitled to due process before a penalty is imposed. (Incidentally, marrying outside the Church is a serious matter for a Catholic, but he or she is not excommunicated for doing so.)
Theologically speaking, it would be wrong to interpret excommunication as a declaration of eternal damnation. Even separated from the community of the Church, no one is beyond the reach of God’s grace and mercy. And, in fact, with few exceptions, the penalty of excommunication can be lifted by any priest in the sacrament of reconciliation, or confession, presuming that the usual conditions for absolution are met, including sincere repentance, and a firm desire not to repeat the sinful behavior.