it is where all the binaries (programs) live (that are not system critical, those would be in sbin). so whenever you execute ls? it is actually /usr/bin/ls and so on and so forth.

then there is the "man" command. basically a manual. you can use it to find out stuff about other commands and such by just typing "man [command]" for example "man ls"

edit: this knowledge has NOT been acquired by RTFM but rather by watching YouTube

Keep in mind this is FreeBSD, but this is the perfect structure that FreeBSD obeys.

Linux distros fuck it up on a daily basis. App devs interpret it themselves and fuck it up even further.

But if you dont know what etc, usr or bin or local means, it should give you a better understanding. Navigating docs, manuals and commands should be easier.

You can look at the Filesystem Hierarchy Standard for starters.
About manuals, try man man in the command line, then something like man ls.

Just type in man to go through the binary manual, also called man page :)