Why A transmission line has a characteristic impedance (commonly 50 ohms for coax used in ham stations), and power flows smoothly down it only when the load it sees equals that impedance. When the antenna feed point impedance differs from the line's impedance, part of the incident wave cannot be absorbed by the load and is reflected back toward the transmitter. That reflected wave interferes with the forward wave to create standing waves, which is what an SWR meter reports; a perfect match gives 1:1 SWR and zero reflected power.
Watch out Operating at resonance only makes the feed point reactance zero, it does not guarantee the resistive part equals 50 ohms, so an antenna can be resonant and still show reflected power. Using a balanced antenna with unbalanced line causes common mode current on the coax shield, a separate problem from reflection, and excess power damages parts rather than creating reflections.
Reflection is a mismatch, not a resonance or a power problem: line ohms must equal antenna ohms.