In kitchen hygiene, cross-contamination often means moving harmful microorganisms from raw food, hands or equipment to ready-to-eat food. A cutting board used for raw chicken and then for salad without proper cleaning is a common example.
Keep raw and ready-to-eat foods separate, wash hands, and clean equipment appropriately. Allergen cross-contact is a related but distinct problem: cooking that kills microbes does not reliably remove allergenic proteins.
Reference: Further reading.