Pentecost commemorates the descent of the holy spirit upon the apostles, Mary the mother of Jesus and other followers of Jesus Christ after the crucifixion and resurrection of Jesus Christ.
It is a day celebrated exactly 50 days from Easter Sunday, ( Easter- a day Jesus Christ resurrected from the dead).
The apostles prayed for nine days in an upper room for the descent of the Holy spirit which Jesus Christ promised them.
On that day, the Holy spirit descended on the apostles like tongues of fire and they began to speak in strange languages as the spirit led them.
In Christian tradition, Pentecost birthed the church of Christ.
That was when the first Christians spoke in strange languages, while people of other tribes heard them in their various languages.
It was the fulfillment of what Jesus promised his disciples, that after He leaves, He would send the Holy spirit which would bear witness to all what Jesus had told and taught them.
The word Pentecost is derived from the Greek word for 50 and originally referred to a feast celebrated 50 days after Passover. ( Source: https://www.ncronline.org)