Theologically the Holy Doors represent the gates of Jerusalem, through which Christ entered on Palm Sunday.
In the Russian practice, there are detailed rules as to when the doors are to be opened during Vespers, Matins, and the Divine Liturgy.
When the gates are opened, it represents moments when God is especially present to his people, such as during the reading of the Gospel, or when the faithful are receiving Holy Communion.
There is also a curtain or veil, scored to remind that in the Temple in Jerusalem, behind the Holy Doors which is opened and closed at specific times during the services.
Alternatively a sliding panel depicting "Christ the Great High Priest" may take place of the doors and curtain and certain churches may have no physical barrier at all.
[4] Only the higher clergy (bishops, priests, and deacons) are permitted to go through the Holy Doors, and even they may only pass through them when it is prescribed by the liturgical rubrics.