The time (and date) stamp, usually of a file, when the event (or last change) occurred.
A timestamp is a sequence of characters or encoded information identifying when a certain event occurred, usually giving date and time of day, sometimes accurate to a small fraction of a second. The term derives from rubber stamps used in offices to stamp the current date, and sometimes time, in ink on paper documents, to record when the document was received. A common example of this type of timestamp is a postmark on a letter. However, in modern times usage of the term has expanded to refer to digital date and time information attached to digital data. For example, computer files contain timestamps that tell when the file was last modified, and digital cameras add timestamps to the pictures they take, recording the date and time the picture was taken.
A timestamp is the current time of an event that is recorded by a computer. Through mechanisms such as the Network Time Protocol (NTP), a computer maintains accurate current time, calibrated to minute fractions of a second. Such precision makes it possible for networked computers and applications to communicate effectively. The timestamp mechanism is used for a wide variety of synchronization purposes, such as assigning a sequence order for a multi-event transaction so that if a failure occurs the transaction can be voided. Another way that a timestamp is used is to record time in relation to a particular starting point. In IP telephony, for example, the Real-time Transport Protocol (RTP) assigns sequential timestamps to voice packets so that they can be buffered by the receiver, reassembled, and delivered without error. When writing a program, the programmer is usually provided an application program interface for a timestamp that the operating system can provide during program execution.
Why is this important?
To quote from a recent question (on AskUbuntu): u1partial - and modification stamp on files
Modified date and time should not be changed unless the filecontent has changed, and should follow files through all kinds of copying.