When something is consecrated it is declared to be sacred or holy. Many Catholics, for example, believe that bread and wine are consecrated, or made sacred during Holy Communion, becoming the body and blood of Christ.
The word consecrated is made up of the Latin suffix con-, which expresses intensive force and sacrare, meaning "dedicate." Something that is consecrated is intensely dedicated to the point of being declared holy. It's not just bread and wine that people believe to be consecrated. You might call a cemetery "consecrated ground."