What’s serializing instruction in Intel terminology?

Serializing instructions force the processor to complete all modifications to flags, registers, and memory by previous instructions and to drain all buffered writes to memory before the next instruction is fetched and executed. For example, when a MOV to control register instruction is used to load a new value into control register CR0 to enable protected mode, the processor must perform a serializing operation before it enters protected mode. This serializing operation ensures that all operations that were started while the processor was in real-address mode are completed before the switch to protected mode is made.