The phrase "to guard against" means to take preventative measures or precautions in order to avoid a particular risk, danger or problem. It implies being cautious and vigilant so as not to fall victim to something negative that could happen. The expression is often used when discussing security measures, such as securing your home against intruders, protecting sensitive information against hackers, or guarding against financial losses by diversifying investments. In essence, "to guard against" means being proactive in anticipating and preventing potential harm before it occurs.