An unlocked phone is one that will accept a SIM card or eSIM from any operator, rather than only the network that originally sold it. Phones bought outright from a manufacturer or an independent shop usually ship unlocked. Phones bought on a carrier plan or instalment deal, which is common in the USA, often start life locked and stay that way until the balance is cleared.
This matters for travel because a travel eSIM is just another operator’s line, delivered as software instead of plastic. If your phone is locked to your home carrier, the eSIM profile may install fine and then refuse to connect: no signal, no data, or an activation error that looks like the eSIM is broken when the real culprit is the carrier lock. An unlocked handset has no such gate, so the profile just works.
Checking is quick. On an iPhone open Settings, then General, then About, and read the Carrier Lock line: it should say No SIM Restrictions. On Android the wording differs by brand, so the surest test is popping in a SIM from a different network, or asking the carrier that sold you the phone. Your device’s IMEI is the number a carrier uses to look up and clear a lock, so keep it handy.
Sort the lock out before you fly, not at the airport, since unlocking can take a few days. If your phone is already locked, our carrier lock fix guide walks through it, and you can confirm the rest on the device compatibility page.