Equivalence Classes with mod 3

I am told that ≡ mod 3 relation is an equivalence relation on the set {1,2,3,4,5,6,7}. But I have no idea what this means or how to find the equivalent classes.

Thank you in advance for everyone’s help

DreamProxies - Cheapest USA Elite Private Proxies 100 Private Proxies 200 Private Proxies 400 Private Proxies 1000 Private Proxies 2000 Private Proxies ExtraProxies.com - Buy Cheap Private Proxies Buy 50 Private Proxies Buy 100 Private Proxies Buy 200 Private Proxies Buy 500 Private Proxies Buy 1000 Private Proxies Buy 2000 Private Proxies ProxiesLive Proxies-free.com New Proxy Lists Every Day Proxies123