Difference between / what is thread safe or non thread safe in PHP

We used to see different binaries for PHP, like non thread or thread safe? What does this mean? What is the difference between these packages?

A Thread Safe version should be used if you install PHP as an Apache module, as a Worker MPM (multi-processing model).PHP has to be thread-safe to be able to play ball correctly with Apache!

The Non Thread Safe version should be used if you install PHP as a CGI binary.