It's all about copyright.
It is illegal to publish other people's stuff without permission, so anything that you submit to this website must be your original work, or you must have permission from the owner to publish it. It's not possible for the site owners to check this so it is your responsibility.
By sending stuff in you are granting permission for it to be put on the site. Copyright remains with you, but we are not responsible if it is copied by third parties.
Similarly, stuff that appears on this site belongs to the person or people that submitted it, and you do not have permission to copy it.