BPM stands for "Beats per minute" and is a measure of the tempo of a song. The higher the BPM is, the faster the tempo is.
Knowing the BPM of songs is crucial if you are a DJ and want to mix different songs together. By mixing songs of the same BPM you can create flawless transitions.
The detected BPM is just an estimation and may differ from the actual BPM of the track. This tool should however point you in the right direction of determining the BPM.