That is completely dependent on how you set up the share. When you share a folder/file you have to specify the access allowed. By default anyone with the link can access it and therefore there is no sign in needed. It means exactly what it says - anyone with the link can use it and therefore no sign in is required. At a minimum you want to probably require a password.
However this is a security risk so if it is sensitive data you really should only give access to certain people. And of course to identify themselves they need to log in.