Metadata is hidden information associated with your file that is a more in-depth description of the data listed with it. This can include your file's Name, Size, Date Taken, Date Modified, Tags, Description, and even the Location it was taken at!
In short, most of the time it's important to keep your file's metadata intact as much as possible so you have accurate information about when, where, and why it was taken.
Here are some more articles regarding your file's metadata: