An 'osteotomy' is an operation where a bone is cut and re-positioned. The 'tibia' is the shin bone. In a high tibial osteotomy the bone of the upper tibia is cut and repositioned.
Usually a wedge is opened up to improve the alignment of the long bones (opening wedge osteotomy). Sometimes a wedge of bone is taken out (closing wedge osteotomy) (see image).
The most common condition treated by high tibial osteotomy is 'unicompartmental arthritis' - where the one half of the knee is damaged by arthritis while the other half is still in good condition.