In the previous passages Jude calls believers to personally and corporately Build up their faith, Pray, Keep in Gods love and Wait patiently in Christ.
By living out those admonitions we continue the sanctification process which ultimately is God transforming us. In the midst of our transformation, or metamorphosis, He 'keeps' or protects us from eternally falling into 'the fire' (Jude :23).
Fantastically enough He doesn't just protect us from eternal wretchedness but Christ presents us to our Holy God free of our sinful nature - our stains and blemishes. Ultimately He does this with an everlasting and contagious joy! All creation groans for this joy.
...to those who have been called, loved and kept.