What does Matthew 28:8 mean?

This is verse includes one of the details that cause the Bible to "ring true" in human experience. These women (Matthew 28:1) have just had an encounter with an angelic being. These creatures are terrifying enough that they almost always have to assure any humans they encounter (Luke 1:11–13; 2:9–10; Acts 10:3–4). This angel appeared in such a fearsome form that a detachment of Roman soldiers fainted at the sight of him. This angel then rolled the large stone from in front of Jesus' tomb.

The angel revealed to the women that Jesus is risen from the dead. The tomb is empty. He has given them a message to pass on to the disciples, and he has told them to do it quickly.

Matthew reports they did leave quickly with feelings of both fear and great joy. That's exactly what you would expect to feel in this moment: fear about the angel and Jesus' missing body, but great joy at the news that He is alive, raised from the dead. The women ran to report all of this to the disciples. They won't arrive, though, before their joy is made whole.