Computer processors can handle millions of instructions each second. Once one instruction is processed, the next one in line is processed, and so on. A pipeline allows multiple instructions to be processed at the same time. While one stage of an instruction is being processed, other instructions may be undergoing processing at a different stage. Without a pipeline, each instruction would have to wait for the previous one to finish before it could even be accessed.
To understand the benefit of a pipeline, imagine that a car manufacturing plant had to wait for each car to be fully completed before starting on the next one. That would be horribly inefficient, right? It makes much more sense to work on many cars at once, completing them one stage at a time. This is what a pipeline in a computer allows. Pipelining, as it is called, often keeps around six instructions at once in the processor at different stages of processing. Pipelines can be used for the CPU as well as for accessing memory (DRAM).