hi! you'll likely get more and more in-depth responses if you could specify which era/map you're referring to, but for a general history of the development of cartography, there are several posts in the FAQ that can get you started:
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The usual answer to this surprisingly common inquiry is to explain that latitude can easily be determined on a clear day by checking the position of the sun above the horizon at noon, or the moon's relation to a star. And longitude can be determined simply using a precise clock, or some other, more complex methods. The parts in between positional observations can be sketched fairly accurately. Sailors can easily move around to observe coastlines, and they have a very strong interest in knowing exactly where they are.
That's how we got accurate coastlines of Europe and parts of Africa by the mid 17th century. A 19th century atlas will show the Iberian peninsula almost as accurately as a satellite image. What's much more difficult is mapping the interior, where heavy vegetation or hills may keep you from seeing very much at any one time.