What does a hard link do?

This has been mentioned in this question here: What's the difference between alias and link?

But I am not even sure what I am looking for. I wish to create a hard link of the type where if a program 'uses' it in the specified directory, it will actually use it from the other central location.

Does something like that exist? For example, would I create ln /path/from/file /path/from/file?

The above command does something like it I believe.

Thank you!