A line joining all points of equal salinity is an isohaline.
In simpler terms, salinity refers to the amount of salt in water, and an isohaline is a line on a map that connects all the points in a body of water that have the same level of salinity. So, if you were to take a water sample at each point along an isohaline, you would find that they all have the same amount of salt in them.
Isohalines are important in oceanography and marine biology because changes in salinity can affect the behavior and distribution of marine organisms, as well as ocean currents and other physical processes.