The verb deō means to bind, tie, or fasten — physically securing a person or object. In the NT it also carries a crucial metaphorical dimension: in the 'binding and loosing' language Jesus gives His disciples, deō refers to spiritual and ecclesial authority — declaring something forbidden or permitted, and engaging in spiritual warfare.
Matthew 16:19 is the cornerstone text: Jesus gives Peter 'the keys of the kingdom of heaven; whatever you bind [deō] on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.' This authority is extended to the whole community of disciples in Matthew 18:18. The background is rabbinic: rabbis 'bound' (prohibited) and 'loosed' (permitted) certain practices in Torah interpretation. Jesus grants His community this same interpretive and spiritual authority, with heaven backing their decisions. In Revelation 20:2, Satan is bound (deō) for a thousand years — the ultimate binding, echoing Jesus' parable of binding the strong man (Matt 12:29). John 11:44 provides an unforgettable image: Lazarus comes out of the tomb bound hand and foot, and Jesus commands, 'Take off the grave clothes and let him go' — a picture of resurrection that does not yet remove all of death's accoutrements until the community acts.