Sisu is a Finnish word meaning passionate perseverance, fortitude, guts, and never giving up. The document discusses how winter is a time when competitors may withdraw into their comfort zones, but this is the time to show sisu and not give up by taking stock, improving yourself and your offerings, and continuing to conduct business aggressively. It encourages the reader to stand up during the winter and say they will not give up and have the determination to get the job done.