CD will change the current working directory (./) of VISTA to be path_name. This working directory will remain current until either you issue CD again, or you exit VISTA. On exiting VISTA, you will be in the *original* working directory from which you executed VISTA.
CD is most useful when you have defined the default image directory to the current working directory, (./), allowing you to navigate among different directories containing data of interest without having to redefine the default image directory each time with the SETDIR command. It also defines the current working directory for shell commands ($).