Load a music file and press Detect BPM. The tool analyses the low-frequency onsets and reports the most likely tempo along with half-time and double-time alternatives.
The track is downsampled and low-pass filtered to isolate the kick and bass, an onset energy envelope is built from it, and that envelope is autocorrelated to find the interval that repeats most strongly. That interval is converted into beats per minute.
For music with a steady drum pattern the estimate is usually within one BPM. Sparse, rubato or heavily syncopated material is much harder, which is why the tool also shows the half-time and double-time candidates for you to judge.
Autocorrelation cannot tell a beat from every second beat, so a 90 BPM track can score strongly at 180 BPM. Compare the reported value with the alternatives shown and pick the one that matches how you would tap along.