Lime is often served on the side of tacos for seasoning.
A squeeze of lime adds acidity, brightness, and aroma to rich meats, fried fillings, seafood, beans, and salsa. The juice can sharpen the flavors of the taco without changing its basic ingredients, which is why wedges are commonly placed beside a plate or included with a street-food order.
Lime is especially familiar in Mexican cooking, where it appears alongside tacos, ceviche, soups, drinks, and many salsas. Different varieties are used, including the small Mexican lime and the larger Persian lime. Their flavor and juiciness vary, but both provide the tart citrus character diners expect.
Lemon is a reasonable culinary substitute, and some regions or restaurants may serve it instead. Grapefruit and orange can also appear in marinades or specialty preparations, but they are not the standard side garnish for an everyday taco. The question asks what is often served, so lime is the single conventional answer rather than an exclusive rule.