Different kind of head of line blocking. In HTTP it refers to a technique where a client sends multiple requests without waiting for their respective responses.

Different kind of head of line blocking. In HTTP it refers to a technique where a client sends multiple requests without waiting for their respective responses. However, the server has to send the responses in order - so if the client makes request A and request B, the response for B can't be delivered until A's response is delivered. So if A's response is large, B will be delayed. In HTTP/2, it's all multiplexed - so the server can interleave responses.