O próprio Apache explica o que esses campos são quando consultados. De este tutorial :
Srv Child Server number - generation
PID OS process ID
Acc Number of accesses this connection / this child / this slot
M Mode of operation
CPU CPU usage, number of seconds
SS Seconds since beginning of most recent request
Req Milliseconds required to process most recent request
Conn Kilobytes transferred this connection
Child Megabytes transferred this child
Slot Total megabytes transferred this slot
The Client, Vhost and Request are the columns that are of the most use. Client is the IP of the person accessing the resource. VHost is the domain or subdomain being accessed. Request is the actual file on the site that is being accessed.