Thieves' tools are a collection of skeleton keys, long metal picks, and assorted other tools that boost a character's open lock skill. The bonus provided ranges from +1 to +12. In order to use these items, a character must have bought at least one rank in the Open Lock skill (as this skill does not allow untrained skill checks).
To use these items, right click the tools in the character's inventory, select the "Use: Open Lock" option, and then click on a locked door or chest. This will initiate an open lock skill check (modified by the tools' bonus). The tools are destroyed after this check, even if the check failed.