Techie version: mod_perl is a Perl module that enables you to embed a Perl interpreter within the Apache web server. Embedding the interpreter allows you to write Apache handlers or modules entirely in Perl. You can even configure the server using Perl code. And all existing modules on CPAN (or elsewhere) are available to you during this process.
English version: mod_perl speeds up CGI scripts and other Perl programs by negating the need for a seperate Perl interpreter. Normally when a website visitor runs a Perl program the request is sent to the server, then from the server to the Perl interpreter, the interpreter compiles the Perl code and runs it, then sends the result back to the server, from where it is sent back to the visitor. Using mod_perl the request simply goes visitor > server > visitor. mod_perl also caches compiled code which can save on server overhead as well as giving marked speed increases.
The security implications are fairly obvious - the server is opened up to direct attacks from within Perl code, attacks from which it is normally shielded (at least to a degree) by the interpreter. It is also at the mercy of sloppy coding, syntax errors, etc etc.
The final nail in the coffin is that mod_perl enabled Apache servers have massive memory demands - anywhere from 5-20x the memory demand of a server running Apache on its own.
Hope this helps,