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Thermal noise (or Johnson-Nyquist noise) is randomness added to a signal passing through any electronic component due to the component's heat. The heat (i.e., movement in the atoms) affects the flow of electricity by agitating the electrons that are passing along the flow of the charge. Thermal noise varies by component design, but at least a portion is always directly proportional to the temperature of the component during operation. The most sensitive radio telescopes depend upon cooling key circuits to cyrogenic temperatures to keep the thermal noise low, so as to allow extremely small signals to stand out sufficiently from the noise.
In electronics (thus, radio astronomy), random modifications to the received signal such as thermal noise, and even the signal itself, are often given in temperature units (noise temperature for a noise source, e.g., antenna temperature).