Core files are files created when an error occurs - someone running an application closes it incorrectly and the system can't send anymore output back to the user - creates a core file (usually in their home directory) - Applications or services can also create core files - I was running console command for a server and created a core file
medusa% file core
core: ELF 32-bit MSB core file SPARC Version 1, from 'cconsole'
medusa% ls -l core
-rw------- 1 tghunter sysadmin 1941504 Jun 7 11:32 core

Another user created one using HP OpenView -
# ls -l core
-rw-r--r-- 1 gmlaroe ncc 1596208 Jun 10 07:02 core
[root@somewhere]:/home/gmlaroe
# file core
core: ELF 32-bit MSB core file SPARC Version 1, from 'ovw'

Note that each of these uses up quite a bit of disk space.

Another area you can look at (because you are running Solaris 8) is all the extra packages that come with Solaris 8.
/usr/apache
/usr/perl5
/var/apache

If you don't need them, they could be removed

If you don't need man pages, remove them by searching for them first (insure you don't care about them) and then remove them.

It is up to you to figure out if you need these files - it's your system and you should know what you do or do not need.