The Traveler's Sack, previously called the Bag, is an item added by Actually Additions. Right-clicking it will open it.
In the GUI, the Traveler's Stack can be configured to automatically pick up items and put them in the sack. It is also possible for a 4-item blacklist or whitelist, with options for respecting metadata, ignoring NBT and ignoring Ore Dictionary entries.
Shift right-clicking the Traveler's Sack on an inventory will empty the Traveler's Sack into that inventory.