Metadata is information stored in almost any type of file. It can include your name, your company or organization's name, the name of your computer, the name of the network server or drive where you saved the file, personalized comments and the names and times of previous document authors, revisions, or versions.
These details could be useful or detrimental to you depending on the situation. For example, if you want to track the different authors of a file it could be good, though if you wish to conceal the different authors of a file it could be bad. If you are working in legal matters or simply wish to maintain privacy, removing as much data as possible can be prudent, as metadata will follow the file if you share a copy of it with someone else.
Metadata can also be very helpful, especially when making searches in Windows. For example, if your collection of pictures has well maintained metadata, then it is very easy to search and find a certain picture you took, at a certain location on a very specific day.