A dummy load is a resistor of 50 Ω, built to handle a transmitter's power, that stands in for an antenna. The transmitter delivers its power into the resistor, which turns it all into heat, so almost nothing is radiated. You use it any time you want a transmitter running without putting a signal on the air.
What it's for
Testing into a dummy load keeps your test signals off the air.
Testing and tuning a transmitter or amplifier without causing interference or being heard by anyone.
Measuring output power with a wattmeter or a scope, with no antenna's SWR affecting the result.
Comparing: if a problem disappears when you swap the antenna for a dummy load, the antenna or feed line is at fault, not the radio.
A known 50 Ω reference for calibrating an analyzer, or terminating an unused port.
Power, voltage and heat
Same power, three ways to state the voltage. A scope shows peak-to-peak; a meter reads RMS.
Across a 50 Ω load the voltage follows from P = V² ÷ R, so V = √(P × R). 100 W gives 70.7 V RMS, which is 100 V peak and 200 V peak-to-peak, with 1.41 A flowing. At 1,500 W it is 274 V RMS and about 5.5 A. These are real voltages, so treat the terminals of a loaded load as live.
The load must also cope with the heat. A rating in watts is for a particular time: some loads are rated continuous, others only for a short on-time in each few minutes. A mode with a steady carrier (key-down CW, FM, RTTY, a long tune-up) puts the full power in constantly, while voice SSB has a much lower average. The rating is usually quoted at a particular frequency and may fall at higher frequencies; check it.
Is it really 50 Ω?
Illustrative shapes, not measurements. A load is only a good 50 Ω on the frequencies it is specified for.
A resistor has a little stray inductance and capacitance, and these matter more as frequency rises. A basic load that is fine on HF can read poorly at 144 or 440 MHz, so its SWR reading is misleading there. Use a load rated for the band in question. Common builds are a high-power resistor on a finned heat sink, or an oil-filled can for longer, heavier duty.