Most locks are only opened by the specific key assigned to them. However, it can sometimes be convenient to have all locks in a set use the same key or be opened by a single master key.
For convenience, many people will buy for example 5 identical locks which all work from the same key. These are known as "Keyed Alike" locks.
A "Master Keyed" lock is a unique lock with its own key but part of a larger set. The whole set then can be overridden by what is known as the "Master-Key". For obvious reasons the master-key should be kept secure as an unauthorized copy would compromise the security of the whole set.