The Witchdoctor robes are a member's only piece of the witchdoctor camouflage gear. It is a reward that can be obtained from the Herblore Habitat by catching all ten normal jadinkos in the same week, then speaking to Papa Mambo. However, Papa Mambo will randomly give out either the robes or the witchdoctor legs for completing this task, so it can take more than a week to obtain one. This requires a hunter level of 80 to complete (which can be boosted).
If you manage to catch all 3 God Jadinkos within the same week, Papa Mambo will grant you a boost on Hunter, Herblore, and Farming experience for 30 minutes for wearing pieces of the witchdoctor camouflage gear. The witchdoctor robes provide a 2% boost, which stacks with the witchdoctor mask (2%) and the witchdoctor legs (1%).
After obtaining them, you can receive another witchdoctor robes the following week, though you also have an even chance of obtaining the witchdoctor legs instead. Note that the Herblore Habitat rewards reset every Wednesday at 0:00 UTC, at which point players have the ability to obtain the other article of the Witchdoctor set.