According to Wikipedia Meta Data is simply “Data about Data”.
For example when I take a picture it would include data about where and when the picture was taken. It will include data about the file size and resolution. It may even include data saying who to the picture.
When you share your stuff on social media, or when companies say they want access to your meta data, its a good idea to keep this stuff in mind. Some companies might even want to sell this meta data in order to sell you advertising.