It's the average load on a resource. Without knowing what that resource is (memory, CPU, disk I/O, network bandwidth) it's kind of hard to be more specific.
All the Apache questions can be answered if you RTFM at
what is are the file system in linux?
The file system on modern Linux distros is
ext3 although the /boot partition is often
ext2. The swap partition is formatted as
swap.
what are directory in linux structure and use of they?
Linux files are organised as a directory tree, starting from the root directory '/'. It's kind of like the Windows structure, except that there are no drive specifiers (C:, D

and the slashes go the other way. Oh, and there are symlinks which kind of act like aliases, and special 'files' that appear in the hierarchy, but aren't actually files at all (like /dev/null). As for what goes where in the directory structure, this is mostly standardised but does vary from distro to distro. You'd have to read the docs on their respective websites, or buy a Linux Systems Admin manual.
You sound like someone who's been dumped with supporting something you know very little about. I think you'll need to do a lot of reading...
Steve
[small]"Every program can be reduced by one instruction, and every program has at least one bug. Therefore, any program can be reduced to one instruction which doesn't work."
(Object:
erlDesignPatterns)[/small]