The interjection idou is an aorist imperative form of eidon (to see), used as a demonstrative particle to arrest the reader's or listener's attention: 'Look at this! Pay attention! Here is something you must not miss!' It appears over 200 times in the NT and roughly 1,400 times in the LXX. It is the Greek equivalent of the Hebrew hinneh.
Idou is the Bible's 'stop and notice' signal, placed before the most astonishing announcements in salvation history. The angel Gabriel opens the Annunciation with idou: 'Behold, you will conceive and give birth to a son' (Luke 1:31). John the Baptist's identification of Jesus is introduced with idou: 'Behold, the Lamb of God, who takes away the sin of the world!' (John 1:29) — one of the most theologically loaded sentences in Scripture. The Risen Christ speaks to the seven churches through idou: 'Behold, I stand at the door and knock' (Rev 3:20); 'Behold, I am coming soon!' (Rev 22:7,12,20). Revelation 21:3,5 combines two idous: 'Behold, the dwelling place of God is with man' and 'Behold, I am making all things new' — the final announcement that demands the whole universe's attention.