Imagine you have a special door that only allows certain people to enter. To decide who gets in, there’s a security guard named RADIUS. Every time someone wants to come in, they tell RADIUS their secret password. RADIUS then checks his list to see if the password is correct. If it is, he lets them in; if not, he says, “No entry!”
So, in plain terms:
A RADIUS server is like that security guard. It’s a system that checks if someone has the right password to access a network or service. When devices or users try to connect (like to a Wi-Fi network), the RADIUS server checks their credentials to decide if they’re allowed. If they are, they get connected; if they’re not, they get blocked. It’s a way to keep networks secure by controlling who can access them.