Ward is a shield above your health that is generated by certain effects and rapidly decays over time.
Ward has no maximum value, but will always decay towards 0 unless you continue to generate more or have a stat called Ward Decay Threshold.
Ward does not decay below your Ward Decay Threshold. For example, having 500 Ward Decay Threshold means that decay will not lower your Ward below 500, and Ward Decay rate at 1000 Ward is halved.
Ward Decays more quickly as you gain more of it, but this decay can be slowed with Ward Retention stat.
Ward retention can be found on items and the passive tree, and the Intelligence attribute also grants Ward Retention.