Remote code execution is the ability an attacker has to access someone else's computing device and make changes, no matter where the device is geographically located.
Vulnerabilities can provide an attacker with the ability to execute malicious code and take complete control of an affected system with the privileges of the user running the application. After gaining access to the system, attackers will often attempt to elevate their privileges.
The best way to protect a computer from a remote code execution vulnerability is to fix holes that allow an attacker to gain access. Microsoft often releases security patches addressing remote code execution vulnerabilities in its monthly Patch Tuesday fixes.