Secondary storage refers to all forms of data storage other than main memory. Unlike main memory, the data on it was not directly accessible to the CPU; data in secondary storage generally had to be brought into main memory through a peripheral device before it can be operated upon.
Over time there have been many forms of secondary storage the are now generally all mass storage: magnetic tape is one of the oldest, and is still in use today. Disks are almost as old, and again, still in use (although the rise of solid state mass storage such as flash memory may make them obsolete).
There are a number of other types which are now obsolete: punched cards, paper tape, etc; these often were a bridge to prior non-electronic information-processing technologies (e.g. paper tape was used in teletypes).