Head-of-line blocking (HOL blocking) in networking is a performance issue that occurs when a bunch of packets is blocked by the first packet in line. It can happen specially in input buffered network switches where out-of-order delivery of packets can occur. A switch can be composed of input buffered ports, output buffered ports and switch fabric.
When first-in first-out input buffers are used, only the first received packet is prepared to be forwarded. All packets received afterwards are not forwarded if the first one cannot be forwarded. That is basically what HOL blocking really is.