I believe it is the number of games that you have lost because you ran out of time.
http://www.chess.com/forum/view/tournaments/you-must-have-a-timeout-ratio-lower-than-10-to-play-in-this-tournament?lc=1
Also, I would recommend using the search box at the top of the chess.com pages to see if anyone posted a simular question and if it has been answered.
Does anyone know what a timeout ratio is? Please comment.