eSIMs are virtual SIM cards that can be downloaded and stored electronically on your phone unlike a physical SIM.
I'm finding these fantastic for travel. There are a few companies that help you buy an eSIM for a network in the country you're going to. You can buy and set up the eSIM in two minutes just before you go. No more struggling after you land in a new country with the crappy airport WiFi, no more having to swap SIMs all the time.
For example, my UK network provider has quite high data rates for roaming in the US ($0.10/MB). So just before my flight to the US I bought a 1GB eSIM from Airalo for about $4.50. It can use both AT&T and T-mobile networks and has worked great so far.