frequency bins are intervals between samples in frequency domain. For example, if your sample rate is 100 Hz and your FFT size is 100, then you have 100 points between [0 100) Hz. Therefore, you divide the entire 100 Hz range into 100 intervals, like 0-1 Hz, 1-2 Hz, and so on. Each such small interval, say 0-1 Hz, is a frequency bin.
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