What is eSIM and what does it do or what’s the difference

It is a digital SIM that is used instead of a SIM card. Some models from the last several years can use either or both eSIM and SIM card while the iPhone 14 sold in the U.S. supports only eSIM (there is no physical SIM slot). Also see: About eSIM on iPhone - Apple Support

iPads are sold in two version -- one that is wi-fi only and one that is Wi-Fi + Cellular. You can purchase a cellular data plan for the Wi-Fi + Cellular model. Also see: iPad + Cellular - Apple