Here is how J. I. Packer answers that question in his essay, “The Glory of God and the Reviving of Religion” in A God-Entranced Vision of All Things (pp. 100-104):
Revival is God touching minds and hearts in an arresting, devastating, exalting way, to draw them to himself through working from the inside out rather than from the outside in.
It is God accelerating, intensifying, and extending the work of grace that goes on in every Christian’s life, but is sometimes overshadowed and somewhat smothered by the impact of other forces.
It is the near presence of God giving new power to the gospel of sin and grace.
It is the Holy Spirit sensitizing souls to divine realities and so generating deep-level responses to God in the form of faith and repentance, praise and prayer, love and joy, works of benevolence and service and initiatives of outreach and sharing.
We need us some of that!