A legacy computer system is an older one. It is one that you are stuck with because replacing it would be too problematic. It would be complex, costly, and it may interfere with specific programs, and whatever protection may be in place.
Whenever you purchase something on credit card, conduct banking transactions, or take flight, computer systems dating back several decades are an integral part of the process. Banks, air traffic control systems, and many civil or military functions depend on technology that most of us would consider obsolete.
For example, the Pentagon's strategic Automated command and control system is part of a fifty -three- year old computer network constructed around the IBM series minicomputer. It was launched in 1976. You would need around four disks to store one average MP3 music file.