From the FAQ of the UK-based Carphone Warehouse[1]:
"What is a SIM free phone?
SIM free phones are sold without any SIM card or network attached to them. Normally, people buy the phone and then choose a SIM only deal for their calls and data. All SIM free phones come unlocked, so you can choose whichever network you like.
What are the benefits of buying a SIM free phone?
Normally, people buy a SIM free phone when they want to own the handset outright and take out a SIM only deal. This reduces the cost of a monthly plan as you are only paying the network for your minutes, texts and data allowance. SIM only deals aren’t only less expensive but they can be more flexible, as 1-month rolling contracts and 12-month contracts are common place."
[1] https://www.carphonewarehouse.com/mobiles/sim-free.html