Difference between meal and mustard in Korean
식사·겨자

식사meal
식사 means a meal, or the act of eating a meal. It is a polite, common word you can use for breakfast, lunch, or dinner. You will often hear it in daily conversation, for example when asking if someone has eaten.

겨자mustard
겨자 means mustard, a strong and spicy sauce or powder made from mustard seeds. People often eat it with hot dogs, sandwiches, sausages, and other foods. In Korean, it can also mean the sharp mustard flavor used in some sauces and dressings.
The difference
식사 means a meal, like breakfast or dinner. 겨자 is something you eat with a meal, not the meal itself.