Crop rotation simply means planting vegetables in different garden locations every 2-3 years, that is, rotating their location in the garden. It's really helpful to have a sketch or diagram of your vegetable garden each year, indicating where each type of vegetable was planted that year. Then, when planning for the following season, you know where to plant the vegetables.
Pests and diseases can build up in the soil over time. Rotating crops moves plant hosts to a new location which helps prevent the spread of diseases. Moving plants also means soil-borne insect pests have a harder time finding plant hosts.
Members of the same vegetable family can be susceptible to the same pests and diseases, so members of the same family follow the same rules. This means, for example, that you should relocate any of the nightshade family (tomatoes, peppers, eggplant, potatoes) in the garden each year or two. Also, for example, don't plant peppers in the same location as tomatoes were planted the previous year.
Aster family - Artichoke, endive, escarole, lettuce, raddichio, jerusalem artichokes
Beet family - Beets, chard, spinach, quinoa
Cabbage family - Arugula, bok choy, broccoli, brussels sprouts, cabbage, cauliflower, kale, kohlrabi, mustard, radish, rapini, turnip
Carrot family - Anise, carrot, celery, cilantro, dill, fennel, parsley
Gourd family - Cucumber, melons, pumpkin, squash, watermelon
Grass family - Corn, barley, rice, rye, wheat
Legume family - Beans, peas, peanuts, fava beans, soybeans, lentils
Nightshade family - Tomato, pepper, eggplant, potato
Onion family - Chives, garlic, leek, onion, shallot
A simple example of crop rotation in a garden that is divided into 4 sections.