What are sysctls?

Sysctl is a means of configuring certain aspects of the kernel at run-time, and the /proc/sys/ directory is there so that you don't even need special tools to do it!

In fact, there are only four things needed to use these config facilities:

a running Linux system

root access

common sense (this is especially hard to come by these days)

knowledge of what all those values mean

As a quick 'ls /proc/sys' will show, the directory consists of several (arch-dependent?) subdirs. Each subdir is mainly about one part of the kernel, so you can do configuration on a piece by piece basis, or just some 'thematic frobbing'.