A Subdeacon is a cleric ( like priests and deacons) but does not receive Holy Orders.
It is a clerical role that existed in Roman Catholicism prior to 1973. Pope Paul VI suppressed it in favor of the lay role of Instituted Acolyte.
But the Eastern Catholic Churches retain that order.
They perform many of the tasks that you might expect from an Instituted Acolyte, but wear vestments, including (in the East) a stole.
The Subdeacon is still a role in the Tridentine High Mass. The EF High Mass requires a celebrant Priest, a Deacon and a Subdeacon.
Those roles are generally filled by Priests or Deacons ( if there are three priests present, one will celebrate Mass, one will vest as a Deacon and read the Gospel, one will vest as a Subdeacon and read the Epistle.
In our parish, we have a man who was ordained (small ‘o’) to the Subdiaconate while he was living in Iraq. In the '60’s he emigrated with his wife to the US, but to an area with no Chaldean parish. So his children were raised as Roman Catholics, while technically being Chaldean Catholics.
After moving to the Detroit area, where there are a large number of Chaldean parishes, his children felt more at home at Roman Catholic parishes. So with the permission of both bishops, he assists at our Masses in his role as Reader and as the equivalent of an Instituted Acolyte. Those he does, as a cleric, wear the alb and stole to which he is entitled.
en.wikipedia.org/wiki/Subdeacon