Lemon juice just as the name implies is the juice from a lemon. When you cut open a lemon and squeeze it the juice will come out. You can find lemon juice already bottled on the shelf in your grocery store. Lemon juice can be used as a marinade for chicken, fish or steak or for lemonadde or as a mixer in drinks. These are just a few of the uses be creative when cooking and see what not only the juice but the zest of the lemon does to bring out the flavors in your food.