This feast commemorates the end of the 40 Days our Lord appeared to his disciples after the Resurrection of his body and opening their minds to understand the Scriptures. He then ascended with his glorified human nature and His divine nature to the right hand of God the Father, as the disciples were instructed to prayerful await the Gift of the Holy Spirit on Pentecost.

See a Youtube video that explains this icon with comparison to Pentecost:

https://www.youtube.com/watch?v=-ny0-cTfu7o&sns=em