An execute buffer is a package of information used to communicate with the 3D device driver. By packing information into an execute buffer the number of calls to the device driver can be minimised. An execute buffer consists of a vertex list followed by an instruction stream consisting of opcodes and data.
Opcode is short for operation code and is an instruction defining how the data following the opcode is to be interpreted. Opcodes may specify how the vertex list is to be lit and rendered, drawn, send changes to the renderer, etc.