[costco]

Tor has something called a consensus that lists all relays and their flags. Clients need this to know which relays to make a circuit with. For most clients, they select a relay labelled as a guard from this file which is where their traffic first enters Tor. Some countries realized they could just block all of these IP addresses and stop people using Tor, so there are unlisted guard nodes called bridges designed for censorship circumvention that you have to get by filling out a captcha or say sending an email.