VPN
a private, encrypted tunnel into a company's network
A VPN, or virtual private network, lets people outside an office connect to the company's internal systems as if they were inside. Traffic travels through an encrypted tunnel across the internet. The device at the company end checks who is connecting and lets them in. That makes it one of the most exposed and most valuable devices a company runs.
Think of it as
A private tunnel from your home straight into the office basement, with a guard checking badges at the office end.