A Travelers hook, sometimes called a Shrum Tool, is used to bypass locks with deadbolts or anti flipper deadbolts. This tool is almost always used on doors that open toward you and where you can reach the doorframe.

The Long version of the Traveler Hook is designed to get past lock plates that try to block this type of attack and is sold separately, you can find it here.

It can also be used to disassemble, re-key, repair or reassemble locks.
All in all, a very useful tool