Visibility in C++ (also called access control or access specifiers) determines who can access (read, write, call) the members (variables, functions, etc.) of a class.
There are three visibility levels in C++:
public
private
protected

TL;DR – One-line versions

public = "everyone can use this"
protected = "family only" (class + children)
private = "my private stuff – nobody else touches it"

These three keywords are the main tools for encapsulation in C++ – one of the core ideas of object-oriented programming.