I am planning to buy an used one for outdoor shooting with my Pentax k-x.Is this a good budget choice for first tripod? http://www.digidirect.com.au/accessories/tripods_and_monopods_/velbon_/photo_tripods_enthusiast/velbon_cx-660
This is an alignment pin. This pin dovetails into a receiving hollow built into the base of most video cameras. The idea is to prevent unwanted camera movement that otherwise might occur when panning. Additionally, when the camera is dismounted and remounted, this pin affords accurate repositioning. Additionally, many video and still cameras are mounted to a tripod via a quick release mechanism. These allow rapid camera attachment and detachment. The locator pin helps insure the remounted camera is accurately aligned as before.
I can answer the title of the question - "What is the purpose of the second bump on the plate of this tripod" but your actual question differs greatly
The bump is designed to prevent or reduce rotation of the camera on the tripod plate. For example, my Canon 80D has ridges on its base which the bump would protrude into and prevent rotation of the camera.
Often they are sprung, to make them adjustable for different cameras