A carrier-locked phone (sometimes "network locked") is restricted by software to accept SIMs only from a specific network. If you bought a phone bundled with a contract, it may be locked to that network's SIMs.
UK rules
Since December 2021, UK carriers are required by Ofcom to ship phones unlocked — so any new contract phone from EE, O2, Three, Vodafone or their MVNOs should already work with any UK SIM.
Older phones
Phones bought before December 2021 on contract may still be locked. You can request an unlock from the original carrier — it's free and usually takes a few days.
How to check
Try inserting a SIM from a different network (or scan a Fuse eSIM QR). If you see "SIM not supported" or "Network Locked", your phone is locked. Otherwise, you're good.