‘Best before’ refers to quality: your food will be at its best before the date given.
After this date, it might not be at its best, but it will still be safe to eat. Use your senses to make a judgement.
‘Use by’ refers to safety: you must not eat food past the ‘use by’ date.
You cannot always smell the bacteria that causes food to spoil, so after the ‘use by’ date, the food may appear perfectly fine to eat, but could still lead to food poisoning.