The earth's crust, or lithosphere, contains many different plates that are moving in different directions. These plates can be colliding, pulling apart, or sliding past one another. Areas where two plates are colliding are called convergent boundaries. An example of a convergent boundary is occurring at the Himalayas, where the Indian plate is essentially crashing into the Eurasian plate. Areas where two plates are pulling apart are called divergent boundaries, an example of which is occurring at the Mid-Atlantic ridge. And lastly, areas where the plates are sliding past each other are called transform boundaries. The San Andreas fault in California is an example of a transform boundary where the Pacific Plate and the North American Plate are sliding past one another.
Because all of these boundaries involve plates moving in different directions, a single line on a map is not enough to inform the user of the direction of movement. Often times maps will include arrows or other symbols to demonstrate which direction the plates are moving. The direction plates are moving will determine the types of geological features found in the area and even the types and intensities of earthquakes that occur along those boundaries. For example, earthquakes along divergent and transform boundaries tend to be shallow, while earthquakes along convergent boundaries can vary from shallow to very deep.
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