This manual is full of passages that tell you what particular keys do. But Emacs does not assign meanings to keys directly. Instead, Emacs assigns meanings to named commands, and then gives keys their meanings by binding them to commands.
Every command has a name chosen by a programmer. The name is
usually made of a few English words separated by dashes; for example,
next-line or forward-word. Internally, each command is
a special type of Lisp function, and the actions associated with
the command are performed by running the function. See What Is a Function in The Emacs Lisp Reference
Manual.
The bindings between keys and commands are recorded in tables called keymaps. See Keymaps.
When we say that “C-n moves down vertically one line” we are
glossing over a subtle distinction that is irrelevant in ordinary use,
but vital for Emacs customization. The command next-line does
a vertical move downward. C-n has this effect because it
is bound to next-line. If you rebind C-n to the command
forward-word, C-n will move forward one word instead.
In this manual, we will often speak of keys like C-n as
commands, even though strictly speaking the key is bound to a command.
Usually, we state the name of the command which really does the work
in parentheses after mentioning the key that runs it. For example, we
will say that “The command C-n (next-line) moves point
vertically down”, meaning that the command next-line moves
vertically down, and the key C-n is normally bound to it.
Since we are discussing customization, we should tell you about
variables. Often the description of a command will say, “To
change this, set the variable mumble-foo.” A variable is a
name used to store a value. Most of the variables documented in this
manual are meant for customization: some command or other part of
Emacs examines the variable and behaves differently according to the
value that you set. You can ignore the information about variables
until you are interested in customizing them. Then read the basic
information on variables (see Variables) and the information about
specific variables will make sense.