The Tallus is a communications device, used by the Timebreakers to communicate with their groups of dimension-hoppers Exiles and Weapon X. The Tallus also enables the team to travel from one dimension to the next. One Tallus was shown to possess a shard of the M'kraan Crystal, and it is assumed that every one Tallus has a shard of the M'Kraan Crystal.
It is usually worn by the leader of the group. Only the wearer of the Tallus can communicate using it. Tallus communication is usually vocal, but it can also be used to manifest holographic projections which can be seen by the wearer. The Exiles' Tallus has been worn by Blink, Mimic, Sabretooth, and Psylocke. The Weapon X Tallus has been worn by Sabretooth and then Gambit before the group was exterminated.
Once the Exiles had assumed control of their missions, the Tallus was used in communication between Heather Hudson and the group. Following Heather's departure (really just a sabbatical), the Tallus was used to communicate between Sage and the team. When Heather returned, she assumed telecommunications control again while Sage was absorbed into the Panoptichron.
The Tallus was shown to be bound to its wearer. This was exhibited when Hyperion ripped off Gambit's arm to get the Weapon X's Tallus, but it reappeared on his other arm. The Tallus also allows the Panoptichron to keep track of its wearer: Psylocke was invisible to the Crystal Palace's sensors until Sabretooth placed a Tallus on her.