This directive controls whether or not Squid prefetches pipelined requests. It is disabled by default, so Squid acts only on one request at a time (per connection). If you enable this directive, Squid processes up to two client requests at once.
Note that the order of responses must match the order of requests. Thus, if the prefetched (second) request completes before the first, it is delayed until the first response is sent.
Squid doesn’t implement pipelining on the server-side. It always opens a new connection to an origin server (or neighbor) if there are no idle persistent connections.