P/B Ratio question for the more experienced traders

I've read that anything under 1.00 is considered a good buy and hold for value traders as it's generally thought to be currently undervalued below 1.00. However, what does it mean when a company has a negative P/B ratio? Is that severely undervalued at that point or am I missing something? I'm watching a ticker that has a -2.65 P/B ratio and it keeps getting more negative as the share price goes up. Any help in understanding P/B more is greatly appreciated.