What is a sliding window?
If you have the samples of a signal of length $L$ (corresponding to 10s analog duration for example) and want to analyse its spectrum, then you can peform an $L$ point DFT/FFT on the whole data at once.

This would provide you the maximum spectral resolution but minimum temporal resolution; i.e., time locality of the events are mainly lost (or better become obscured in the phase spectrum).

If you wish to sacrifice some spectral resolution but increase the temporal instead, then you can analyse the signal in small blocks (overlapping chunks) and move the block window in time to observe the change in the local spectral behaviour. This motion of the analysis window is referred to as sliding action. And the analysis is essentially a sliding window analysis, aka short-time Fourier analysis, windowed Fourier analysis etc.

Note that for various reasons it's best to use some weighting within windowing before the DFTs are applied.