Prefork and worker are two type of MPM apache provides. Both have their merits and demerits.
By default mpm is prefork which is thread safe.
Prefork MPM uses multiple child processes with one thread each and each process handles one connection at a time.
Worker MPM uses multiple child processes with many threads each. Each thread handles one connection at a time.
For more details you can visit http://codebucket.co.in/apache-prefork-or-worker/
and for apache prefork settings you can read following blog http://codebucket.co.in/apache-prefork-mpm-configuration/