The domain is every input a function uses; the range is every output it produces. Two bars on the graph — one along the bottom, one up the side — worked through a weight-over-years example and a temperature-over-a-day example.
A function connects inputs to outputs. Domain and range are just the names for those two collections — every input you used, and every output you got back.
The domain is the complete set of input values used. The range is the set of output values those inputs produced.
Picture it as two bars on the graph: one running along the bottom marking every input covered — that’s the domain — and one running up the side marking every output reached — that’s the range.