Carb loading
Carbohydrate loading is eating more carbohydrate than usual in the day or two before a long endurance event, while training less, to fill your muscles' glycogen stores. It helps in events lasting longer than about 90 minutes.
How much and how long
Sports nutrition guidelines suggest around 10–12 grams of carbohydrate per kilogram of body weight per day for the 36–48 hours before the event — for a 70 kg athlete, about 700–840 g a day. That's far more than most people expect, so it usually means swapping some fibre and fat for easy carbohydrates rather than simply eating bigger meals.
Common mistakes
Trying new foods, eating too much fibre and loading for a short race are the usual errors. Expect to weigh a little more afterwards — glycogen is stored with water. Practise your race-week eating before a long training session first.