What does Acts 12:7 mean?

Peter is in a Roman prison in Jerusalem. James, the brother of John and son of Zebedee, has already been beheaded by Herod Agrippa I (Acts 12:1–2). The Feast of Unleavened Bread is coming to a close, and Peter's death is imminent. Peter is chained at the wrists—the term used in Greek includes the hand, as well, thus the confusion over where Jesus received nails during His crucifixion. Peter is guarded over by four soldiers, two of whom presently surround him. And he's sleeping (Acts 12:1–6). He's sleeping hard enough the angel has to nudge him to wake him up—the Greek terms used here don't always imply a forceful blow.

Peter knows he will eventually be crucified (John 21:18–19), but he's not excessively bothered by it. His attempts on his own behalf in the past, first by attacking a servant in the mob who arrested Jesus and then by trying to hide his involvement, led to nothing but shame (John 18:10–11, 15–18, 25–27). After His resurrection, Jesus sought Peter out specifically (1 Corinthians 15:5; Luke 24:34; Mark 16:7). Later Jesus asked Peter three times if he loved Him, each time responding by commissioning Peter to care for and lead His people (John 21:15–17). Peter has seen Jesus calm a storm that threatened to kill him, Jesus, and the other disciples (Mark 4:35–41). He's already been released from prison by an angel once (Acts 5:19). He is content to live or die, whichever will best serve those who need to hear Jesus' offer of redemption.

In the New Testament angels usually take on the role of messenger. The angel Gabriel tells Zechariah about the birth of his son John (Luke 1:11–17, 19) and tells Mary about Jesus (Luke 1:26–38), and an angel (quite possibly Gabriel) tells Joseph about Jesus (Matthew 1:20). One will tell Paul that he will survive a storm at sea and spread Jesus' message in Rome (Acts 27:23–24). Occasionally, angels minister more directly, notably by comforting Jesus after His temptation in the wilderness (Matthew 4:11). Angels are "ministering spirits sent out to serve for the sake of those who are to inherit salvation" (Hebrews 1:14), and some people today have seen them (Hebrews 13:2). Many others have been rescued or served by angels but will never know.