As a single person you’re not vowed to someone or something else, which means your free to commit yourself to serving God and the church. Certainly we’ll all have times in our life where we’re single; but some people commit the whole of their lives to being single in a heroic gesture of love for God and church.
Most of us at some point in our lives will live the single vocation. This is a time when you haven’t made a formal commitment to a vocation (i.e. ordination or marriage). This is an important time for discernment, personal development and self-understanding. This is also a time of being available to serve: God, the Church and others. For some, this is a permanent state of life and it’s important to recognize that this is a legitimate way of being able to live and serve in the world.