The word tart has two distinct meanings:

1. A small pastry with a shallow crust filled with fruit, custard, jam, or other sweet ingredients. Picture a little pie without a top crust: that's a tart. Fruit tarts might be filled with strawberries, blueberries, or lemon custard. The famous Portuguese pastéis de nata are small custard tarts that have been enjoyed for centuries. Unlike a deep pie, a tart sits in a shallow pan and shows off its filling.
2. Having a sharp, slightly sour taste that makes your mouth pucker. Lemons are tart. Green apples are tart. A tart flavor is the opposite of sweet: it's zingy and makes you wake up and pay attention. You might describe homemade lemonade as pleasantly tart if it has just the right amount of lemon juice. The word can also describe someone's manner of speaking: a tart reply is sharp and biting, like when someone responds to a question with an impatient, cutting remark.