To add to the other answers, the ‘amplitude/power of a signal at a given frequency’ has to do with frequency-amplitude decomposition of a signal.
The frequencies correspond to the frequencies of some periodic functions which we choose for a given decomposition. The set of these functions is the basis of the decomposition.
When the functions are multiplied by the relevant amplitudes, their superposition gives us the original signal.
To add to the other answers, the ‘amplitude/power of a signal at a given frequency’ has to do with frequency-amplitude decomposition of a signal.
The frequencies correspond to the frequencies of some periodic functions which we choose for a given decomposition. The set of these functions is the basis of the decomposition.
When the functions are multiplied by the relevant amplitudes, their superposition gives us the original signal.