I really liked this post at NLM which you can read in full over there:
The practice is so common that many people might be under the impression that this hymn is integral to the Roman Rite. In fact, it is not. To use this hymn as the entrance is a substitute – the last option permitted in the rubrics, presumably only to be used when no other choice is possible – for the authentic Introit, which is Ecce Advenit.
The text translates as "Behold the Lord the Ruler is come: and the Kingdom is in His Hand, and power, and dominion. Give to the king Thy judgment, O God: and to the king’s Son Thy justice."
There is no better time to begin this orienting process than at the very start of Mass. Replacing the proper with a hymn delays this orienting posture until later – and thereby subtly inhibits us from coming together in directed prayer.