MinSpareServers, MaxSpareServers, StartServers, and MaxClients are standard Apache configuration directives that control the number of servers being launched at server startup and kept alive during the server's operation. When Apache starts, it spawns StartServers child processes. Apache makes sure that at any given time there will be at least MinSpareServers but no more than MaxSpareServers idle servers. However, the MinSpareServers rule is completely satisfied only if the total number of live servers is no bigger than MaxClients.
MaxRequestsPerChild lets you specify the maximum number of requests to be served by each child. When a process has served MaxRequestsPerChild requests, the parent kills it and replaces it with a new one. There may also be other reasons why a child is killed, so each child will not necessarily serve this many requests; however, each child will not be allowed to serve more than this number of requests. This feature is handy to gain more control of the server, and especially to avoid child processes growing too big (RAM-wise) under mod_perl.
These five directives are very important for getting the best performance out of your server. The process of tuning these variables is described in great detail in Chapter 11.