That means that the server forks itself a number of times to answer simultaneous requests. The children are isolated from each other.
Threads are similar to lightweight processes.
Threads can share common data. If a thread misbehaves it can corrupt other threads and the server as a whole can go down. On the other hand, the thread model allows for faster, leaner webservers. Apache 2.0 brings the best of both worlds, allowing the user to define number of processes and number of threads per process. Apache 2.0 introduces APR, the Apache Portable Runtime, which increases even more Apache's portability.