An eSIM is a SIM card built into your phone. Instead of putting in a small plastic card, you install a digital profile and your phone connects to a local network. In Dominica that network is FLOW, one of the two main carriers on the island.
You buy the plan online, install it before you fly, and switch it on after you land. Your home number and apps stay exactly where they are. The eSIM only handles your data while you travel, so you can keep WhatsApp, your bank app, and everything else working as normal.