A reader of the New Testament would not hesitate to answer that, referring to Acts 6:1-4, which first mentions the deacon’s role:

In those days, as the number of disciples grew, the ones who spoke Greek complained that their widows were being neglected in the daily distribution of food, as compared with the widows of those who spoke Hebrew. The Twelve assembled the community of the disciples and said, "It is not right for us to neglect the word of God in order to wait on tables.  Look around among your own number, brothers, for seven men acknowledged to be deeply spiritual and prudent, and we shall appoint them to this task.  This will permit us to concentrate on prayer and the ministry of the word." This proposal was unanimously accepted by the community.

A deacon, then, is a servant.  That's what the word means in Greek and that's what deacons are for: to serve.