Cross-contamination is the transfer of germs that cause food poisoning from one surface or food to another. This happens either from food to food, equipment to food and food handler to food.
Prevent the risks of cross-contamination by ensuring that RAW food is stored away from COOKED or Ready-To-Eat food (segregated and stack accordingly), all food contact surfaces (utensils, equipment) are cleaned and sanitize before and after use, and all food handlers are strictly following personal hygiene rules (hand washing, wearing clean uniform).
Do not use same knife to cut raw meat, poultry or fish then cut salad for example without cleaning and sanitizing the knife, and never store raw dripping food in a fridge on top of ready to eat food like salad.