When an RF source's output impedance differs from that of the load, it is said to be a "mismatch." The extent of the mismatch can be characterized in terms of the Voltage Standing Wave Ratio (VSWR). Clink here for a short article on Practical Guidance on VSWR & Return Loss
RF amplifiers, transmitters and generators typically have a nominal output impedance of 50 Ω, and ideally would only be used in applications where the load impedance is also 50 Ω. This ideal situation results in maximum power transfer from the transmitter to the load, so that all power is absorbed in the load with no power reflected back to the transmitter. Unfortunately, RF sources are used in "real life" applications and encountering a pure 50 Ω load is indeed rare.
Bird has designed rugged, calibrated mismatch loads that can be used to test amplifiers, transmitters and generators in real life conditions. Follow this link to Bird's Calibrated Mismatch Loads.