A dual SIM phone is one that has the capacity to run two active phone lines on one device, which you can switch between. This is usually in the form of one eSIM and one physical SIM, although it can be two of the same type (just be sure to check handset spec).
The benefits of having a dual SIM phone is that you can have two numbers without needing two phones. This is useful if you want a separate work and personal number, of if you’re travelling and want to pick up a temporary SIM, local to your destination, to save money.