Welcome to the forum.
A long, long time ago, when the Mass was celebrated only in Latin and with the priest turned toward the high altar instead of facing the people:
* A high Mass was a sung Mass celebrated on Sundays and holy days, that usually lasted well over two hours. The priest sang all his parts from the book that was moved at various points in the Mass, from one side of the altar to the other; and the choir (positioned in the choir loft in back of the Church) chanted all the parts of the congregation, usually with organ or string accompaniment.
There would be more than two altar boys assisting the priest in the sanctuary, carrying the holy water container where the priest dipped his sprinkler, reciting prayers in Greek, ringing the bells, moving the book from one side to the other, lighting the incense, assisting the priest as he washed and dried his hands, and carrying a paten and a lighted candle as they assist at Communion. The people knelt at the communion rails to receive the Body of the Lord.
* A low Mass was a recited Mass, usually the daily Mass or a Sunday evening Mass, where there was no singing, and the priest recited his prayers in a low voice. The people followed the priest's Latin Bible-reading and prayers by silently reading the English translations from their books. Or they silently prayed the Rosary, stopping only when the bells rang for the Consecration. It was much shorter than the high Mass.
But that was more than 40 years ago (may be before you were born.) Since then, the manner in which we celebrate the Mass has changed. First of all, Latin is very seldom used now. Mass in most parts of the U.S. is either in English or Spanish. In the average parish, there would be no sprinkling of holy water on Sundays except during Easter time.
The priest marches up the aisle with a coterie of liturgical ministers (the lectors, altar servers, etc.) while the congregation sings the opening song with the choir. The priest faces the people. The priest hardly ever sings his parts now.
On Sundays and holy days, we now have four Readings from the Bible: the first usually from the Old Testament, then we chant a psalm, another reading from a Letter of St. Paul, and then we stand up to hear the Gospel proclaimed. A homily naturally follows the Gospel proclamation.
You almost do not hear any bells now, and the people walk up the aisle to receive communion standing. No paten is used during communion and no lighted candle, either. But there's plenty of singing.
Today a daily Mass usually lasts 30 minutes, even with one or two hymns or none at all. A Sunday Mass lasts one hour with a lot of singing by the choir. But there is no more distinction between a "high Mass" and a "low Mass."
But no matter. It is still the Mass - the Sacrifice of Christ on the Cross. We Catholics believe that at Mass, we are transported back to Calvary and to the Empty Tomb to witness the Lord's passion, death, and resurrection, to unite ourselves in the offering of His Sacrifice, and to receive His resurrected Body and Blood at Communion.
The is the same one Sacrifice that happened once in history, but is made present to us in an unbloody manner, so we get to be with the Lord in person just as the apostles and the Blessed Mother did at the foot of the Cross.
It's awesome. I pray that one of these days, you and the other Christians would join us as members of the original Christian Church and celebrate Mass with us.
Thanks for asking, and God bless you.