"Servant of God" is a term used by the Catholic Church to refer to an individual who is in the early stages of the canonization process. This is the first step toward possible sainthood.
A person is declared a "Servant of God" when the Catholic Church opens a cause for his or her canonization, usually due to a longstanding reputation for holiness and virtue among the faithful. This phase of the process allows for an initial investigation into the person's life, teachings, and works to be conducted.
During this stage, the person's writings are examined for orthodoxy, and witnesses are interviewed to learn about the individual's reputation for holiness and virtue.
If the person is deemed to have led a life of "heroic virtue," the Church can declare him or her to be "Venerable," which is the next stage in the process of canonization. Beyond this stage are beatification (when the person becomes known as "Blessed") and, finally, canonization (sainthood).
It's worth noting that the title "Servant of God" does not confer any special status within the Church and does not imply that the individual will eventually be declared a saint. Rather, it is simply a recognition that the process of investigation is underway.