Sea level maps are visual representations of how high the sea is at various locations around the globe, how this is changing over time, and how this interacts with the shape of coastlines. Sea level maps are especially useful for illustrating climate change.
Sea level is a measure of the height of the ocean's surface. Scientists generally use an average of the height taken at different times of day, since the surface of the ocean is very mobile and shifts with the tides. This measurement is called Mean Sea Level. Sea level maps use Mean Sea Level to give a mostly accurate representation of what coastlines currently look like or what they would look like if Mean Sea Level rose or fell.
Maps such as these can show current sea level trends, especially in reference to modern climate change or ancient sea levels, as is the case with paleontological maps displaying sea level markers for various dig sites.