A typical 27 MHz transmitter has a coaxial output connector to which you connect a whip antenna. The connector is used to test the transmitter before the antenna is connected.
Various RF devices may be designed with a symmetrical (two-prong) output or input, most have the unsymmetrical (coaxial) input and output. The coaxial ports are typically specified with the impedance of 50 Ohms most test equipment have.
The best conditions to transmit power from a source (transmitter) to a load (antenna) require power matching, or, that both sides have 50 Ohms impedance.