HTTP/2 Multiplexing
Carrying many concurrent request/response streams over a single TCP connection, removing the need for multiple connections per origin.
HTTP/1.1 handles one request at a time per connection, so browsers opened six connections per origin and developers invented sprite sheets, domain sharding and file concatenation to work around it.
HTTP/2 interleaves frames from many streams over one connection. Those workarounds become unnecessary and several become actively harmful — domain sharding in particular defeats multiplexing by forcing multiple connections again.
Also gains header compression (HPACK), which matters because headers are repetitive and often larger than small responses, and server push, which proved unhelpful in practice and has largely been removed.
The limitation to know: it still runs on one TCP connection, so it inherits TCP head-of-line blocking under packet loss.