If linear trend appears in the signal (i.e. the whole signal rises or falls monotonously), it can affect the calculation of low frequency components in the FFT. With the help of the “Signal tools/Remove linear trend” option, you can transform the origin signal to a new signal without linear trend. That signal can be used for further analysis. Linear trend will be removed by subtracting the signals linear least squares approximation from the original signal.

If you want this operation to be performed before each FFT calculation, you can set it in Spectral analysis defaults.