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:
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'.
Sysctl Explorer is an initiative to facilitate the access of Linux' sysctl reference documentation. This is a work in progress and you may consider this increment as a Minimum viable product (MVP) version.