A transmitter should never be keyed up without a load. Normally the antenna is the load but when testing or tuning the transmitter the antenna should not be attached to prevent radiation of a signal unnecessarily. The dummy load replaces the antenna when the transmitter needs to be keyed while turning the RF signal into heat and thus not cause interference.
Every ham shack should have a dummy load many have their antennas on a coax switch with one section of the switch going to the dummy load thus they are able to switch in the dummy load tune the transmitter and switch back to the antenna trim the adjustments and transmit. This makes it easy to any necessary adjustments to the transmitter and quickly switch it back on line. Even if using an antenna tuner if the antennas are near the right frequency and 50 ohms this process should allow a minimum tuning time live on the air.
Most modern transmitters do not need much adjustments before they are ready to transmit especially when there are no band switching involved. But it has not always been that way. The class C vacuum tube amplifier used for AM, FM, and CW usually need to be adjusted as the transmitter was moved across the band. It might take a few seconds to tune the driver, adjust the drive, peak the grid, dip the plate, and go back and readjust each until the proper settings were obtained.
Fifty years ago it was not uncommon for a ham to use a light bulb as a dummy load. We chose a light bulb of the proper wattage, 50 watt bulb for a 50 watt transmitter, 100 watt bulb for a 100 watt transmitter, and so on. The light bulb dummy load could radiate a lot of RF. I have actually worked stations several miles away using just a light bulb.
While the light bulb was okay to use on most CW, AM, or FM transmitters built before 1970 they were never recommended for SSB. It should only be used on transmitters using a vacuum tube finals and never on transistor final amplifier units. They are inexpensive and easy to construct but I personally do not recommend them for modern transmitters or transceivers.
Another dummy load that I played with but until recently I had no idea that anyone else had tried is the salt water dummy load. It really is quite simple
just put two probes connected to a piece of coax in a jar or bottle of pure water. Then add salt until it reaches 50 ohms DC as measured with a VOM. You now have a dummy load. Be sure that such a device is vented to release gas build up and I might add this is deadly (chloride) and explosive (hydride) gases mixed with sodium and oxygen.
Some dummy loads are 50 ohm resistors immersed in oil to dissipate the heat. Other are dry dummy loads which are 50 ohm resistors usually having a heat sink with fins to dissipate the heat. Either way all a dummy load is just a 50 ohm resistor and thus they are rather easy to construct.
A two watt 50 ohm resistor will fit rather nicely inside the barrel of a PL-259 coax connector designed for RG-8U coax. Just run one lead through the hole for the center conductor and solder it; clip it flush. Then bend the other lead in such a way that it can be soldered to the barrel and trim the excess and solder.
The two watt resistor inside a PL-259 can be used safely up to five watts. The connector will act as a heat sink. If a higher wattage dummy load is needed an oil filled dummy load can be constructed inside an empty paint can. Heathkit called this type of load a Cantenna. The oil that is recommended is either transformer oil (electric transformer not automotive transformer) which is expensive and hard to find, or mineral oil which is much less expensive and fairly easy to find but will not conduct heat as well. I saw one construction project that suggested motor oil. To build such a load just use the old formulas for series and parallel resistors. Every resistor should be the same wattage and the same type of construction so dissipate equally and have the same characteristics when heating up. They should have a safety pop out valve so oil will not spill out when transported but will pop out if the oil starts to boil which is better then the can exploding or even the top of the can flying off.
When building a dummy load never use wire wound resistors. The wire wound resistor is constructed just as its name implies. They are made by wrapping a resistant wire around a form. This means it is a coil or inductor and thus adds inductive reactance to the load changing the overall impedance which should be 50 ohm. The higher the frequency the higher the reactance.