An ‘embedded-SIM’. Basically the phone can just imagine it has a normal SIM in a second slot, leaving the actual slot empty, or removing the need for a slot altogether.
It’s good for roaming if the phone’s unlocked, you can have your home SIM as an eSIM and use a local normal SIM in the slot if you don’t have inclusive roaming on your home plan.
Problem is that the network you want an eSIM for has to support issuing them - of which in the UK, only EE does for phones. Once it’s in (on) the phone, you don’t have to worry about the roaming network supporting it - it just looks like you’re connecting normally to them, no difference