A message pod was a device used by the Time Lords as, essentially, a secure courier for delivery of messages or small objects to a specific recipient.
The pod would only open when touched by the person for whom the message was intended. Much like the outer plasmic shell of a TARDIS, the exterior of the pod was nearly invulnerable to outside forces. The message pod would usually pre-program its destination into a nearby TARDIS. (TV: The Mutants) It could be connected to a TARDIS' time rotor to control it. (PROSE: Old Flames)
The Time Lords used a message pod to send the Third Doctor on a mission to Solos to deliver a set of ancient tablets to Ky, a Solonian resistance figure. It contained engravings of Solonian writing. (TV: The Mutants)
Iris Wildthyme also had a message pod, but she never bothered to deliver it. The Fourth Doctor used it to capture Lady Huntingdon. He used Iris Wildthyme's TARDIS to do a particle reversal, pulling her inside the message pod. (PROSE: Old Flames)