Limes are one of the cornerstones of Thai cooking. You will find lime juice in nearly as many recipes as you find fish sauce. In fact, one of the most common and versatile sauces (nam pla prig) is just lime juice, fish sauce and sliced hot pepper. In Thailand, nam pla prik is as common a complement to a meal as salt and pepper is in the US.
If you only have lemons available, you can substitute the lemon for lime, but limes are what are used in Thailand and give a much richer flavor.