Lemons are a bright yellow citrus fruit originating from Asia used for culinary and medicinal uses. Lemons are available in many varieties including Meyer lemon which is less tart and is actually a hybrid and the Eureka Lemon is the tart lemon typically found in grocery stores.
Lemons are so widely in cooking the many uses are too numerous to list but typical uses are for beverages, dressings, desserts, and in general to increase acidity levels in a dish to balance flavors.
If you don't have fresh lemon you can, under some circumstances substitute1 tablespoon bottled juice per tablespoon fresh needed* or try one of these alternatives:
OR> 3 tablespoons of brewed lemon tea
OR> 1 tablespoon fresh lemon balm per lemon needed
OR> 2-3 leaves of chopped lemon verbena per lemon needed
OR> Lime juice (very different flavor profile, works well in Mexican cooking)
*Bottled lemon juice is not always an advisable alternative to fresh but it really depends on how it is being used.