Diabetes? I presume you have been diagnosed with diabetes. A blood sugar reading before meal time will tell your physician (and you) how well your blood sugar is being controlled. Right after a meal your blood sugar will go up normally and is not a good guide. Your physician may request a 2 to 4 hour after meal blood sugar too to see how well your body regulates sugar after a meal.
Pre-prandial checks. Taking readings after meals will naturally be high, due to the food a person just ate, so tracking levels for deciding the best treatment or changes that may be needed in dosing are best to get before meals.