‍An icon is a holy image which is the distinctive art form of the Orthodox Church. An icon may be a painting of wood, on canvas, a mosaic or a fresco. Occupying a very prominent place in Orthodox worship and theology, icons depict Christ Our Lord, Mary the Theotokos, the saints and angels. They may also portray events from the Scriptures or the history of the Church, such as the Birth of Christ, the Resurrection, or Pentecost.

The icon is not simply decorative, inspirational, or educational. Most importantly, it signifies the presence of the person depicted. The icon is like a window linking heaven and earth. When we worship we do so as part of the Church which includes the living and the departed. We never loose contact with those who are with the Lord in glory. This belief is expressed every time one venerates an icon or places a candle before it. Orthodox churches have icons not only on the iconostasis but also on the walls, ceilings, and in arches. Above the sanctuary in the apse, there is very frequently a large icon of Mary, the Theotokos and the Christ Child. The Orthodox Church believes that Mary is the human person closest to God. This very prominent icon recalls her important role in the Incarnation of Jesus Christ. The icon is also an image of the Church. It reminds us of our responsibility to give birth to Christ's presence in our lives.

Iconostasis

The icon of Christ the Almighty, the Pantocrator, is on the ceiling or in the dome. This icon portrays the Triumphant Christ who reigns as Lord of heaven and earth. Looking downward, it appears as though the whole church and all of creation comes from Him. Looking upward, there is the sense that all things direct us to Christ the Lord. He is the "Alpha and the Omega" (Rev. 22:13), the beginning and the end of all. This is the message of Orthodoxy.‍

‍The iconostasis is the panel of icons which separates the sanctuary from the nave. Its origin is in the ancient custom of placing icons on a low wall before the sanctuary. In the course of time, the icons became fixed on a standing wall. In contemporary practice, the iconostasis may be very elaborate and conceal most of the sanctuary. Or, it may be very simple and open in accordance with more ancient custom.

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Source: https://www.goarch.org/-/what-to-expect-when-visiting-an-orthodox-christian-church?inheritRedirect=true