To be always reforming
For some, to be "reformed" means to be a part of a church or denomination that subscribes to a certain creed, like the Three Forms of Unity or the Westminster Confession or the London Baptist Confession. For others, to be "reformed" is more a statement about one's particular theological convictions. Those distinctions can at times be helpful and at others be a hindrance. The word's origin actually has its roots in Christian history.
The Reformation was a historical movement of the church during the 16th century which resulted in the Protestant form of the Christian faith. To be reformed is to happily consider oneself as part of that historical tradition and the principles it recovered and emphasized. Here are a few good resources:
Reformation: A History by Patrick Collinson
The Unquenchable Flame by Michael Reeves
Why the Reformation Still Matters by Michael Reeves & Tim Chester
The Reformation happened because of the leadership of a few key figures, who ended up leaving a rich legacy of life and doctrine. Those who consider themselves to be reformed greatly appreciate these men, looking to them as godly examples (Hebrews 13:7) and making much use of their theological writings. These men chiefly include: Martin Luther (and his student Philip Melanchthon), John Calvin (and his student Theodore Beza), John Knox, and Thomas Cramner.
The Reformation highlighted key essential beliefs of Christianity. To be reformed means one follows and closely adheres to the doctrines the Reformation emphasized. These things include a conviction that salvation is by grace alone, through faith alone, in the person and work of Christ alone, revealed in Scripture alone, all for the glory of God alone. These "five alones" are popularly known as the "five solas". It's a God-centered orientation which cherishes the truths of election and predestination as being critical to the Christian faith.
Tim Challies wrote a great piece showing the place of Reformed doctrine within Christian theology and how it compares with other ideas and movements:
Reformed creeds, like the Three Forms of Unity, the Westminster Confession or the London Baptist confession essentially provide a clearly articulated rallying base for the principles of reformed doctrine. There are others but those three are the most well known and most subscribed to.
To be Reformed is not only a past tense reference but an ongoing present reality. Those who subscribe to reformed doctrine believe that remaining sin is a present ongoing reality for Christians (the doctrine of total depravity) for which the gospel is designed to address. Thus to be Reformed means that a person's heart and life is in a process of ongoing personal reformation.
To be reformed then means to consider oneself as part of the historical movement of the Reformation, to subscribe the main theological tenants of The Reformation AND to be a person who endeavors to ever change and grow into the likeness of Christ.
For further study, R.C. Sproul has written a great book titled What is Reformed Theology.