Metadata is data about data, it can be used to identify what something is. A filename for example is metadata, it tells you what the file is or what the file contents could be. Metadata can be found everywhere, such as in photos, documents, and videos. Metadata doesn't just have to be a filename. When it comes to photos it could be the location the photo was taken, with documents it could be the author of the document or the settings that were used to create that document.
When sharing files, an adversary may be able to analyse them if they are shared publicly or if they are intercepted in transit. For example, if a photo contains metadata about where it was taken, an adversary can use that information to locate you. Documents contain metadata too, documents may contain metadata that can let an adversary infer what device was used to create that document or identify who the author of the document is.
Metadata is everywhere your connections to cell towers from your phone are all logged, that is metadata, and it can track you and be used for a lot of different purposes. How often you talk to someone over the internet is metadata, who you talk to is metadata, the title of your files is metadata. Metadata is hard to avoid, but it can be significantly reduced or spoofed if you manage to pay attention to how it is created or identify where it exists.
You can remove metadata from files using a popular command line program called mat2 or if you are using Tails OS you can use the built-in Metadata Cleaner application.
Removing metadata using mat2
Removing metadata using the Metadata Cleaner in Tails OS