It's called a seatpost. It shouldn't slip if it's a good fit, if it's not a good fit get one that is - if the slot in the seat tube, by the pinch bolt, narrows toward the bolt then the seatpost is probably too small - open the slot with a large flat screwdriver or similar lever and fit a larger seatpost. If the seatpost is too loose but the next size up is too tight you might need to get the seat tube reamed. If the ears the pinch bolt goes through are squeezed in or the bolt is bent then it's been overtightened - straighten the ears, replace the bolt. Some seatposts will just not play nice - if you've covered all the previous points and it still slips, use carbon assembly paste, which is basically gritty grease, to stop the sliding. Some people recommend using lapping compound, which is also gritty grease but primarily designed as an abrasive rather than grip paste - I'd hesitate to use it anywhere near bearings, but bottom brackets are mostly cartridge bearings so should be immune to grit falling down inside the seat tube.