Head-of-Line (HoL) blocking is when the item at the front of a queue can't be processed, stalling every item queued behind it.
In HTTP it occurs at two layers — the application layer and the transport (TCP) layer — and where you solve it is exactly what distinguishes HTTP/2 from HTTP/3.
Application-layer HoL: HTTP/1.1 processes requests serially per connection, so a slow response to an earlier request makes later requests wait on that connection. HTTP/2's multiplexing interleaves streams and removes this.
Transport-layer HoL: but HTTP/2 still rides a single TCP connection, and because TCP guarantees ordering, while it retransmits a lost packet every byte after it (i.e. every stream) is withheld from the application. Logically independent streams get bound together by one lost packet. QUIC, over UDP, manages ordering and retransmission per stream, so packet loss on one stream doesn't block the others. That's why QUIC/HTTP/3 is especially advantageous on lossy mobile and high-latency networks.