The pipeline is a technique used to design the architecture of microprocessors.
Briefly: processing of information data often consists of performing a succession of different operations on input data. Therefore to optimize their performances, microprocessor cores are designed as blocks of cascaded elementary functional units, each one fed by the output of the previous unit: at every cpu clock the unit N process data outptut of unit N-1. The main reason for this choice is that simple processing blocks are able to run faster than complex ones, so pipelined structures allow the design of digital circuits working at GHz clock frequencies, with the throughput performances of nowadays modern processors.
This is (more or less) what is called pipelined architecture.
Due to this HW structure you will find the term pipeline also referred to SW, in the sense that the compiler translates SW instructions in the best way to fill the processor pipeline, decomposing the execution of complex instruction in a succession of elementary operations performed by the HW pipelines stages.
Hope that my explanation was helpful for you.