How can an isoline map show elevation on a flat surface?

The isolines are lines that are used on maps for different purposes. They basically connect dots of same values, which can be height, depth, temperature, air pressure... These lines are used on maps as mentioned, and some of them are specialized for representing heights on flat surface, these are the contour lines. The contour lines connect dots that have the same elevation. The contour lines that have lower elevation tend to be wider, while the ones with higher elevation tend to narrower. Also, as they end, there are special signs, such as dot with number next to it, which represents the height point of the place, thus giving us clue where the highest part is, and where the lower parts are. The contour lines themselves are often marked with numbers, which makes things easier for the people using the maps.