What does Mark 16:10 mean?

Although this verse is not original to Mark's writing, its content is corroborated in John 20:18. Mark 16:8 says that the women talk to no one when they leave the tomb. That is, they talk to no one while they hurry back to the place the disciples are staying. Once they arrive, they tell the disciples the tomb is empty (John 20:2). Mary Magdalene follows Peter and John back to the tomb (John 20:3–9). After they leave, Jesus appears to her, and she returns and tells the disciples (John 20:11–18).

Falsely, some claim Jesus meant for Mary Magdalene to be the leader of the disciples after His ascension. Part of that claim is that this passage teaches she is "Apostle to the apostles."

An apostle is "one who is sent." Specifically, it means one who is sent out by Jesus to spread the gospel. In the church, the term is used both as an office and a role. Jesus sends out the twelve disciples (Matthias is included in Acts 1:12–26) specifically to teach the gospel of salvation. These twelve are the foundation of the church (Revelation 21:14). Others hold the role of apostle, including Paul (Romans 11:13) and Barnabas (Acts 13:2).

Mary may be considered an apostle only under the second definition: as a role, not an office of authority. Jesus does not send her to preach to the world but to witness to His disciples in a specific place and time. She had an important role in Jesus' life, but He did not choose her to lead after Him.