BPM means Beats Per Minute. When creating a custom note chart it is the most important setting. If your BPM is incorrect, then your notes will not line up with the song very well. The higher the number, the faster the notes will fly onto the screen.

You can use the following websites to determine the BPM of your song:
http://www.all8.com/tools/bpm.htm (Play your song, then tap the beat on the keyboard using any key. This site will tell you the average BPM that you'll probably want to set.)

If you find yourself struggling to keep mouth movements in sync with the singing, just double the BPM to gain more flexibility. (e.g. 98 x 2 = 196 Beats Per Minute)